Category Archives: Fertility

An early beta hCG test is a good predictor of an ongoing pregnancy

That first glimpse of two pink lines–is it real?–and your heart start to pound with excitement. You’re pregnant!

But after a few minutes of celebration, you descend back to earth. Okay, you’re pregnant, but for how long? Will this pregnancy stick? This is a new, more hopeful limbo than the much bemoaned two-week wait, but it is still not picnic.

These kinds of worries are inevitable. Miscarriage is very common, especially early in pregnancy. And for most women, good info about viability does not come until the first ultrasound usually performed at 8-10 weeks.

Fortunately, women undergoing fertility treatments receive information about their chances a bit earlier with a “beta”–that is, a blood test of their beta hCG (Human Chorionic Gonadotropin) levels.

What is HCG (Human Chorionic Gonadotropin)?

The embryo produces hCG as it burrows into your uterine lining. HCG passes into your bloodstream and helps to maintain the uterine lining and keep progesterone levels high in early pregnancy.

During the first trimester, blood levels of HCG rises quickly, normally doubling every 48 hours in early pregnancy, until reaching a peak around 20 weeks, after which they begin a slow decline.

Predicting ongoing pregnancy with HCG

The level of hCG in your blood predicts your chances of an ongoing pregnancy–which researchers usually define as one that lasts through the first 8-12 weeks.

The table below summarizes findings from several IVF clinic-based studies which tracked pregnancy outcomes by hCG levels. These numbers apply to singleton pregnancies. Twin pregnancies tend to have higher hCG levels.

Screen Shot 2018-05-16 at 1.09.19 PM

When comparing your test results, pay close attention to the post-retrieval or post-transfer date listed in the Day tested column. Some studies report by day from egg retrieval (post retrieval). Others report from day after blastocyst or embryo transfer (post transfer). HCG rises rapidly in early pregnancy, so whether a specific beta is a positive or negative signs depends on precisely when hCG was measured.

The hCG threshold for likely viability rises with each day. As one research team that examined hCG levels on different days post-retrieval reported “hCG samples … were taken on day 14, 15 or 16 after oocyte retrieval in 204 patients undergoing IVF or ICSI were analyzed… optimal cut-off levels to discriminate between viable and non-viable pregnancies… were found at 76, 142 and 223 IU/L for day 14, 15 and 16, respectively”

Interpreting your betas

If your betas are above the numbers listed in the table above, congratulations! Your pregnancy has a very high chance of continuing through the first trimester.

Note, however, that HCG is not as good of an indicator of a live birth as it is of avoiding a first trimester miscarriage–so you’re not completely off the worry hook yet. Your first ultrasound will provide better information than your beta about your chances of a live birth.

What if your hCG is below those in the above table? Don’t panic. In all of these studies about 40-60% of the pregnancies with values below the stated thresholds were ongoing. In other words, hCG was better at predicting a good outcome than it was at predicting  a bad outcome. Because hCG levels vary a lot from pregnancy to pregnancy, there is no strict cutoff for determining viability.

HCG Levels during the first trimester

Normal HCG levels range widely in early pregnancy.

Below are the ranges of beta hCG by week following your Last Menstrual Period (LMP), according to the American Pregnancy Association.

  • 3 weeks LMP: 5 – 50 mIU/mL
  • 4 weeks LMP: 5 – 426 mIU/mL
  • 5 weeks LMP: 18 – 7,340 mIU/mL
  • 6 weeks LMP: 1,080 – 56,500 mIU/mL
  • 7 – 8 weeks LMP: 7, 650 – 229,000 mIU/mL
  • 9 – 12 weeks LMP: 25,700 – 288,000 mIU/mL
  • 13 – 16 weeks LMP: 13,300 – 254,000 mIU/mL
  • 17 – 24 weeks LMP: 4,060 – 165,400 mIU/mL
  • 25 – 40 weeks LMP: 3,640 – 117,000 mIU/mL

Serial hCG measurements

What does often indicate an impending miscarriage, however, are hCG levels that fail to double every 48 hours or that drop over time. This nearly always indicates a failing pregnancy. It can also indicate an ectopic pregnancy–an pregnancy that has implanted somewhere other than the uterus.

One final caveat: All of the above studies involved women undergoing IVF. We cannot say whether these numbers apply to women undergoing IUI or who conceived naturally.

Special case: Frozen embryo transfers

Some but not all studies find that HCG levels are lower and less predictive of miscarriage after frozen (as opposed to fresh) embryo transfers.

Screen Shot 2018-05-17 at 12.07.56 PM.png

In Xue’s 2014 study, nearly 100% of pregnancies that made it past 12 weeks had day 12 hCG levels about 50 IU/L, as shown in the table below. But many miscarriages also had hCG levels above this threshold; 50 IU/L did not discriminate well between ongoing pregnancies and miscarriages.

A higher threshold of 135-147 IU/L performed better. Nearly 94% of pregnancies with hCG levels above this threshold were ongoing.

Have you had a beta? What was it and how did your pregnancy turn out?

(If you have had a first trimester ultrasound, you may wish to check out my post on miscarriage risk by week, by fetal heart rate, and by other risk factors like your age.)

References

Porat S, E. al. Early serum beta-human chorionic gonadotropin in pregnancies after in vitro fertilization: contribution of treatment variables and prediction of long-term pregnancy outcome. https://www.ncbi.nlm.nih.gov/pubmed/17307176

Kim, J. H. et al. Serum biomarkers for predicting pregnancy outcome in women undergoing IVF: human chorionic gonadotropin, progesterone, and inhibin A level at 11 days post-ET. Clin. Exp. Reprod. Med. 39, 28 (2012).

Kim, Y. J. et al. Predictive value of serum progesterone level on β-hCG check day in women with previous repeated miscarriages after in vitro fertilization. PLoS One 12, (2017).

Kumbak B, E. al. Serum oestradiol and beta-HCG measurements after day 3 or 5 embryo transfers in interpreting pregnancy outcome. https://www.ncbi.nlm.nih.gov/pubmed/17007661

Papageorgiou TC, E. al. Human chorionic gonadotropin levels after blastocyst transfer are highly predictive of pregnancy outcome. https://www.ncbi.nlm.nih.gov/pubmed/11704121

Xue Y, E. al. Effect of vitrification versus slow freezing of human day 3 embryos on β-hCG levels. https://www.ncbi.nlm.nih.gov/pubmed/24880883

Ochsenkühn R, E. al. Predictive value of early serum beta-hCG levels after single blastocyst transfer. https://www.ncbi.nlm.nih.gov/pubmed/19878087

Lambers MJ, E. al. Optimizing hCG cut-off values: a single determination on day 14 or 15 is sufficient for a reliable prediction of pregnancy outcome. https://www.ncbi.nlm.nih.gov/pubmed/16466846

Sung N, Kwak-Kim J, Koo HS, Yang KM. Serum hCG-β levels of postovulatory day 12 and 14 with the sequential application of hCG-β fold change significantly increased predictability of pregnancy outcome after IVF-ET cycle. Journal of Assisted Reproduction and Genetics. 2016;33(9):1185-1194. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5010803/

Prenatal Testing: Download my ebook (free)!

Who needs prenatal testing for genetic disorders? How much does your risk of carrying a baby with a genetic disorder increase with your age? Which prenatal test or screen is right for you?

These are among the questions my amazing co-author Molly Dickens (and fellow pregnant scientist blogger/partner-in-crime) and I tackle in our new ebook on prenatal testing. We provide a quick “cheat sheet” on how these tests compare, and then dig into the nitty-gritty details of each as well as the history of prenatal testing and how to estimate your personal risk of carrying a baby with a genetic disorder.

Even having gone through prenatal testing twice before, I was still surprised to learn while researching this book that…

  1. Testing only women over 35, as was the case in the 80s and early 90s, misses around 70% of the cases of Down’s Syndrome.
  2. The risk of miscarriage from amnio and CVS is around 1 in 1000–far, far lower than the still commonly cited but outdated 1 in 100 estimate.
  3. Until you are 38 years old, you are more likely to carry a baby with a chromosomal disorders involving tiny pieces of chromosomes than carry a baby with disorder involving missing or extra whole chromosomes–but these disorders are not well detected by any prenatal screens. This is a huge problem, because younger women are often advised to get screens over diagnostic testing.
  4. How women under 35 are more than twice as likely to get a false alarms on prenatal screens as women over 35.

And so much more that I am excited to share.

We all need to up-to-date, evidence-based information to make the best decisions for us and our families. But with prenatal testing rapidly evolving, sometimes doctors and other healthcare professionals are woefully behind or outright misinformed on the latest developments. So please download and share with fellow parents-to-be!

Download the pdf here: Prenatal Testing ebook.

Overwhelmed by prenatal genetic testing options? Download my ebook!

Who needs prenatal testing for genetic disorders? How much does your risk of carrying a baby with a genetic disorder increase with your age? Which prenatal test or screen is right for you?

These are among the questions my amazing co-author Molly Dickens (and fellow pregnant scientist blogger/partner-in-crime) and I tackle in our new ebook on prenatal testing. We provide a quick “cheat sheet” on how these tests compare, and then dig into the nitty-gritty details of each as well as the history of prenatal testing and how to estimate your personal risk of carrying a baby with a genetic disorder.

Continue reading Overwhelmed by prenatal genetic testing options? Download my ebook!

A Threatened Miscarriage, a Subchorionic Hematoma, and How United Airlines (Nearly) Ate My Baby

Three years ago, sitting with my 15-month-old son and my husband during a long layover, on our way back home from Norway, I felt a sudden gush of warm blood.

So much for my miracle pregnancy, I thought. I was only six weeks along and certain I was miscarrying.

Our return trip was already off to a poor start. United Airlines had cancelled our original flight from Newark home to San Francisco. Then, to rub salt in the wound, they refused to refund our first-class tickets.

We never fly first class, but had made an exception for this trip. We were travelling overseas with my 15-month old son and facing a 9-hour jet lag. The chance of sleeping on route, we decided, was worth the extra cash. We had bought our tickets a year in advance to lower the cost.

But United, being United, told us we would have to request a refund, and then maybe they would grant it. And–perhaps just for kicks–they refused to let us access the first-class lounge while in Newark, because our replacement tickets were now in Economy.

My husband–who nevers argues with anyone behind a counter and hates it when I do–spent half an hour arguing with their “customer service” that they should give us passes to the first class lounge. After all, we had paid for first-class tickets even if we no longer had them. But no dice.

So there we were, exhausted and enraged. I had not slept in over 24 hours. My son, sick for the last 3 days with a high fever, had nursed continuously the entire flight from Oslo to Newark. And then I started bleeding.

At that point, I contemplated tweeting, “United, you ate my baby,” but decided against publicly sharing my pregnancy or what I assumed was an impending miscarriage.

The pregnancy had been a surprise, but a welcome one. We had taken over a year to conceive my son. This time we had not been trying. I was still breastfeeding, and my cycles had only resumed a month earlier. We were planning to wait a few more months and then start trying for #2, expecting that it could easily be another 6 to 12 months before we conceived.

The bleeding tapered off by the next morning. I had no pain or cramping, so little fear of an ectopic pregnancy. And I still felt pregnant: nauseated, tired, and lightheaded.

I called my OB,  but they could not fit me in for another 5 weeks. Until then, they told me, just sit tight. Oh, and assume that I was still pregnant, because a miscarriage would have caused several days of heavy bleeding.

I found another OB.

My new OB ran tests. My HCG levels were normal, but my progesterone was low, perhaps because of the nearly constant breastfeeding, the lack of sleep, and the stress. She could not say for sure. She prescribed progesterone supplements for the rest of my first trimester.

Progesterone helps build up and maintain the uterine lining for implantation of the fertilized egg. High levels of progesterone are required to sustain an early pregnancy. But taking progesterone supplements during the first trimester to prevent a miscarriage is controversial.

Over half of miscarriages result from chromosomal abnormalities, and no amount of progesterone will save these pregnancies. A 2013 review of randomized trials, however, found that while progesterone supplements did not alter the risk of miscarriage for pregnant women as a whole, they did significantly lower the chances of miscarriages for women with 3 or more prior miscarriages.

And for women like me, with a threatened miscarriage (defined as any bleeding within the first 20 weeks of pregnancy), who have more than double the normal odds of a miscarriage*, progesterone supplements appear to cut the risk of miscarriage in half, and oral progesterone, as opposed to suppositories, may be especially effective.

So, although I will never know for sure, my OB may have saved my pregnancy.

A Subchorionic Hematoma

At that initial visit, she also performed an ultrasound. The fetal heartbeat was loud and clear, fast and reassuring, racing along like a rabbit’s. When I heard my baby’s heartbeat, I fully exhaled for the first time in days.

Less reassuringly, the ultrasound revealed a subchorionic hematoma–a blood clot next to the placenta and the cause of my bleeding.

Pregnancies with a subchorionic hematoma are considered high risk. They have a higher risk of miscarriage (17.6% versus 8.9%), stillbirth (1.9% versus 0.9%), and placental abruption (3.6% versus 0.7%). They have a slightly higher risk of preterm delivery (13% versus 10%) and for the waters breaking before labor starts (tv-style labor).

The risk varies by the location of the hematoma. Pregnancies with recurrent bleeding or with hematomas located between the placenta and the uterine wall (retroplacental hematomas) have a higher risk of miscarriage and other pregnancy complications like placenta abruption. Because of the risk of placental abruption, bleeding in the second and third trimesters require immediate medical attention.

As worrisome as these statistics sound, most subchorionic hematomas resolve on their own, as mine eventually did. By 11 weeks, we could no longer see the hematoma on an ultrasound. And thankfully, rest of my pregnancy was uneventful. I gave birth to healthy baby girl, who in a few months will turn 3.

Do you have a story of bleeding in early pregnancy? Was a cause detected, and how did things turn out?

Footnote

*About 20% of women experience bleeding during early pregnancy. Figuring out their chances of a miscarriage is far from simple.

One commonly cited statistic states that roughly 50% of these women eventually miscarry. Some digging reveals that this claim derives from a 1981 obstetrics textbook rather than recent research. (Lots of researchers cite papers that cite papers that cite this textbook, and I am willing to bet that none of them have read the original research behind this claim.)

If bleeding starts after detection of a normal fetal heartbeat, most prospective studies find a much lower rate of miscarriage, of 3.4-5.5%.

References

Haas DM, Ramsey PS. Progestogen for preventing miscarriage. Cochrane Database of Systematic Reviews 2013, Issue 10. Art. No.: CD003511. DOI: 10.1002/14651858.CD003511.pub3.

Nagy S, Bush M, Stone J, Lapinski RH, Gardó S. Clinical significance of subchorionic and retroplacental hematomas detected in the first trimester of pregnancy. Obstet Gynecol. 2003 Jul;102(1):94-100.

Sotiriadis A, Papatheodorou S, Makrydimas G. Threatened miscarriage: evaluation and management. BMJ : British Medical Journal. 2004;329(7458):152-155.

Trop I, Levine D. Hemorrhage during pregnancy: sonography and MR imaging. AJR Am J Roentgenol. 2001 Mar;176(3):607-15.

Tuuli MG, Norman SM, Odibo AO, Macones GA, Cahill AG. Perinatal outcomes in women with subchorionic hematoma: a systematic review and meta-analysis. Obstet Gynecol. 2011 May;117(5):1205-12. doi: 10.1097/AOG.0b013e31821568de.

Back Sleeping During Pregnancy and the Sydney Stillbirth Study

Pregnancy can be cruel. Just when you are at your most swollen, bloated, and exhausted, sleep proves frustratingly elusive. Every night, you toss and turn, trying to find a comfortable position, your back aching, and your belly pressing down on your bladder. And then, as you finally start to drift off, you realize you need to pee.

To make matters worse, despite having an enormous bowling ball attached to your stomach, you are told you cannot sleep on your back:

“After 16 weeks of pregnancy, experts advise women to not sleep on their backs, but rather should lie on their sides, ideally the left side.” – mamalette

Who came up with this idea?

This advice stems three studies that have linked back sleeping with late stillbirth (pregnancy loss after 28 weeks). (Interestingly these warnings predated the three studies, so they are not exactly the reason women are told to avoid back sleeping)

I described the first two studies, one conducted in Ghana, the other in New Zealand, in an earlier post, and concluded that not only did they provide no reason for alarm, they certainly do not justify blanket advice again back sleeping.

In 2015, a third study came out linking back sleeping with late stillbirth. Does it change the overall picture?

Continue reading Back Sleeping During Pregnancy and the Sydney Stillbirth Study

Nine Science-Backed Tips for Getting Pregnant Quickly

Back when my husband and I decided to try for a baby, I remember feeling so impatient. I basically wanted a baby right then. Today. That instant. The inevitable nine months seemed too long to wait, let alone the time it would take for us to conceive.

I am sure I am not alone in this sentiment.

So, if you too are feeling impatient to become pregnant, here are some science-based tips to help maximize your chances.

1. Have sex at the right time. Timing intercourse correctly is the most important thing you can do to conceive quickly. Continue reading Nine Science-Backed Tips for Getting Pregnant Quickly

Age and the Risk of Miscarriage

As women, we hear a lot about the difficulty of getting pregnant as we age, but staying pregnant can often be the bigger challenge, especially as our fertility begins to wane.

The risk of miscarriage rises as a woman ages, with a dramatic rise starting after age 37, with the steepest increase occurring after age 40. By age 45, less than 20% of all recognized pregnancies are viable.

The man’s age matters too. Having a partner over the age of 40 significantly raises the chances of a miscarriage.

Over half of miscarriages are caused by genetic abnormalities. As women age, chromosomal defects in their eggs become increasingly common. On average, a woman in her early 20s will have chromosomal abnormalities in about 17% of her eggs; this percentage jumps to nearly 80% by a woman’s early 40s. And as men age, chromosomal defects and point mutations–changes to a single nucleotide in their DNA–become increasingly common.

How The Chances of Miscarriage Vary By Age Continue reading Age and the Risk of Miscarriage

Lies, Damned Lies, and Miscarriage Statistics

Trying to figure out your chances of miscarrying? Sadly, you are going to have a hard time finding good information. 

Many websites claim to tell you your risk of miscarriage, citing statistics that look like these:

Screen Shot 2015-08-25 at 12.50.51 PM

But problems abound with their numbers.

Problem 1: These sites rarely provide their sources, so you cannot tell whether their information is reliable.

Problem 2: These sites do not breakdown miscarriage risk by other known risk factors, like the mother’s age.

Problem 3: Nearly all these sites derive their statistics from just two small studies, one which tracked 222 women from conception through just the first 6 weeks of pregnancy, and another which tracked 697 pregnancies, but only after a fetal heartbeat had been detected–a key point, because heartbeat detection dramatically lowers the chances of a miscarriage.

The lack of good information frustrated me when I was pregnant, and I bet it frustrates you too. So I have compiled a summary of the best research on risk of miscarriage. Where possible, I break down the risk by…

Edit: I also have a new post on how morning sickness signals a lower risk.

Continue reading Lies, Damned Lies, and Miscarriage Statistics

How Egg Freezing Success Rates Change with Age

A few months back, a friend asked that I write about egg freezing:

“As a single woman in my mid-30s who has always been a strong maybe on kids (with preference for yes with right partner/financial circumstances), I’m now in a place where I feel like I need to start planning for either children and partner or freezing my eggs or SOMETHING before the options run out in the next few years… But most people online seem to be writing personal horror stories with multiple IVF fails.”

Many women face a similar quandary, wondering if they should freezing their eggs before it’s too late. Here’s what you need to know about egg freezing to make an informed decision.

Continue reading How Egg Freezing Success Rates Change with Age

The Fertility Cliff at Age 35 is a Myth

Several years ago, before I was married or had even begun dating my husband-to-be, I was chatting with a reproductive endocrinologist about when I needed to worry about my fertility going into decline. I was about to turn 30. Should I be worried? And how many quality reproductive years did I have left?

She told me most women were fine at 30 or 35. At her clinic, she said, she rarely saw women with problems related to “advanced ovarian age” before they turned 37 or 38.

I was surprised, to say the least. Like so many women, I had heard ad nauseam about “getting pregnant after 35.”

Despite all the chatter, I was not actually clear on why 35 was an important cutoff. Was it because getting pregnant was more difficult after 35? Or staying pregnant became challenging after 35? Or was that the age when the risk of chromosomal abnormalities like Down’s syndrome rose dramatically?

It turns out that none of these reasons are correct. Because in fact there is no reason; age 35 is not actually a cliff. It is not even a sharp bend in the curve, a point at which birth rates go into a steep decline. Those sharp bends come later, after 37, and again after 40.

So why has age 35 been etched into our consciousness? Continue reading The Fertility Cliff at Age 35 is a Myth

Fertility in Your 30s and 40s: 7 Things You Need to Know

One of my former colleagues became pregnant her first shot out of the barn, the first month off the pill. Her story would hardly be noteworthy, except that she was 41 at the time.

She wanted to tell other women about her experience, she confided to me. She saw it as a sign that women can have children after age 40.

I simply nodded in response, while I privately wondered if she had not just been very lucky.

But–and this is key–how lucky?

Having a baby in your 30s and early 40s–and earlier, for that matter–is always a chance event. There will be outliers. Some women will give birth naturally at 44. Some women will suffer from early menopause at age 30. But outliers tells us little about the norm.

Anyone who wants to play the conception game, especially if they are postponing childbearing, needs to put anecdotes aside and try to grasp the actual odds. Here’s what every woman needs to know:

Continue reading Fertility in Your 30s and 40s: 7 Things You Need to Know

Does Giving Birth “Reset” a Woman’s Fertility?

Do couples have an easier time getting pregnant after they have already had a child?

I’ll confess, my interest in this topic is personal. We were one of these couples. We took over a year to conceive my son, but our second was a surprise.

Back when my first was born, as we were getting ready to head home after three long days in the hospital, with round-the-clock wake ups, I made the mistake of telling our delivery nurse that we were not planning to use birth control.

She immediately launched into a lecture that we needed birth control. “Giving birth can reset your fertility,”  she stated matter-of-factly. And then added sternly that we needed to start using birth control as soon as we resumed having sex.

Although she briefly made me feel like an errant teenager, I did not take her advice very seriously.

Various reputable sources of medical information, such as WebMd, state that the prior births do not “reset” a woman’s fertility,asserting that the notion is a myth.

Two recent studies, however, suggest there might be something to this idea after all.

Kenneth Rothman of Boston University, led a prospective study, which followed 2820 Danish couples who were trying to conceive for up to 12 cycles.

Rothman then calculated how the woman’s age affected a couple’s fecundability ratio–a statistical estimate of a couple’s ability to conceive each menstrual cycle.

Couples in which the woman had given birth before–about half of the couples in their early 30s and two-thirds of those in their mid to late 30s–had much higher fecundability throughout their 30s:

Screen Shot 2015-02-03 at 12.27.21 PM

Continue reading Does Giving Birth “Reset” a Woman’s Fertility?

It’s (Probably) Safe to Sleep on Your Back While Pregnant

My third trimester in my second pregnancy was rough. The days were fine, but the nights were awful. I could not fall asleep. I was too uncomfortable. And as a second time mom, I was desperate. Months of sleep deprivation were my certain future. Pregnancy was supposed to be an opportunity to stock up on sleep before the newborn period.

The only remotely comfortable position was lying on my back, propped up with a couple of pillows. But several pregnancy websites and and my OBs had warned me against sleeping on my back during pregnancy.

More than anything, I wanted to disregard this advice. But I needed to know how big a risk, if any, I would be taking by sleeping on my back.

My OBs were not helpful in this regard. Within the same clinic, one OB told me to avoid lying on my back from 4 months on, another told me to avoid this position from 7 months on, and a third said not to worry until the last month of pregnancy. When asked, none of them could tell me the magnitude of the risk.

Eventually, I dug into the research myself. Once I did, I understood why the advice is confusing to mothers: the underlying research is a mess.

The rationale for this prohibition is simple enough: lying on one’s back can cause supine hypotensive syndrome, sometimes known as aortocaval compression syndrome.

A pregnant woman woman’s belly can compress the inferior vena cava, a large vein running under the right side of her uterus; and compression of the inferior vena cava can cause a drop in blood pressure. In rare cases, the drop in blood pressure is severe enough to reduce heart output, lower oxygen going to the brain, and cause fainting.

Although the drop in blood pressure is unlikely to harm the mother, the concern is that if a pregnant woman’s oxygen levels drop, her baby’s might too. Under normal circumstances, though, women typically become uncomfortable and change their position before their blood pressure takes a serious dip.

lateral versus supine

Supine hypotensive syndrome has been reported as early as the second trimester, but it is mainly a problem of late pregnancy, after 36 weeks or so.

Despite how scary this sounds (“I might be depriving my baby of oxygen without knowing it”), according to a recent research review, back sleeping is safe for the vast majority of pregnant women. The reviewers build a compelling case: First, only very small percentage of pregnant women experience low blood pressure when lying on their back. Even among those women, the changes in their blood pressure do not appear to affect the fetus. Studies have found no effects on fetal blood flow or on fetal well-being during non-stress tests.

Second, the symptoms of low blood pressure (dizziness, nausea, a rapid heartbeat) are easily recognizable. Women can figure out for themselves if lying on their back makes them uncomfortable, and avoid the practice if it does. In the reviewers words:

Advising women to sleep or lie exclusively on the left side is not practical and is irrelevant to the vast majority of patients. Instead, women should be told that a small minority of pregnant women feel faint when lying flat. Women can easily determine whether lying flat has this effect on them, and most will adopt a comfortable position that is likely to be a left supine position or variant thereof.

Third, previous research did not examine back sleeping. The research only addressed positioning women during surgery, when they are completely immobilized and unable to change their position.

This review was written in 2007. Its conclusions are clear and reassuring. Unfortunately, since its publication, two more recent studies muddy these waters a bit.

The first study was conducted at a maternity ward in Ghana. Two hundred twenty women who had recently given birth reported their sleep practices during pregnancy. Compared to women who slept in another position, the 21 women who reported either sleeping on their backs or “backs and sides”, had higher rates of NICU admissions (36.8% vs 15.2%) and stillbirths (15.8% vs 3.0%), and were more likely to have given birth to an underweight baby (36.8% vs 10.7%). Even when the researchers controlled for the mother’s age, number of children, gestational age, and pre-eclampsia, these differences remained statistically significant.

The second study was conducted in New Zealand. Researchers interviewed 155 women who experienced unexplained late stillbirths (after 28 weeks) about their sleep position both before pregnancy and in the last month, the last week, and the last night before their pregnancy ended. Their responses were compared to 301 control women, who were a similar number of weeks along but with ongoing pregnancies.

The researchers carefully controlled for several known risk factors for stillbirth: obesity, smoking, low socioeconomic status, maternal age, and number of prior children. Even so, sleeping on one’s back the night before corresponded to a higher risk of a late stillbirth compared to sleeping on one’s left side.

In fact, sleeping on one’s right side or in any other position than on the left side correlated with a higher risk of stillbirth.

Considered together, these two studies seem reason for caution, but not fear. They have a number of problems. Both were quite small, and both relied on women’s ability to recall what positions they slept in. And for the Ghana study, it’s unclear how the findings translate to women in a high income country.

Even assuming these findings hold up, the absolute risk appears to be very, very low. In the New Zealand Study, during its 3-year study period, the rate of late stillbirth was 3.09/1000. The researchers estimate that left side sleeping would lower the risk to 1.93/1000, whereas right side or back sleeping would raise it to 3.93/1000.

To put this risk further in perspective, the New Zealand study also found that going to the bathroom an average of once a night or less (as opposed to two or more times) was associated with an increased risk of a stillbirth. This magnitude of the increase was comparable to back sleeping. Yet, based on these data, no one has proposed that pregnant women should wake up more often to go to the bathroom.

So, what are we to make of these data? In my personal opinion, the research is not strong enough to support blanket warnings against back sleeping. Yes, there is a plausible mechanism for back sleeping causing problems. But the bulk of the evidence suggests that compression of the vena cava very rarely causes problems.

Depending on her risk tolerance and ability to sleep, one woman might look at these data and feel fine sleeping on her back. Another might choose to sleep exclusively on her left side. Both seem like reasonable decisions.

For me, sleeping with a pregnancy pillow, resting mostly but not completely on my back was the right choice. In part, this was because I wanted to be conservative: A tilt of 10 degrees (which you can obtain by propping up your right side with a pregnancy pillow or a regular pillow) has been shown to reduce the risk of low blood pressure.

For me, sleeping in with my right side slightly propped up felt pretty safe. But mostly, it felt comfortable.

Did you avoid lying on or sleeping on your back during pregnancy?