Morning Sickness & Miscarriage: How Much Does Nausea Lower Your Risk?

For most women, the first trimester is brutal. About a week or two after you find out you’re pregnant (two lines!), you start to feel exhausted, queasy, and like you might throw up at any moment.

You are not alone. Somewhere between 70-80% of pregnant women experience nausea during their first trimester. About half of women also experience vomiting.

Nausea tends to worsen with each subsequent pregnancy, and become more likely to persist into the second trimester. Your race and ethnic background also matter: White women are more prone to nausea than Black and Asian women, and Black women are more likely to have nausea that starts after the first trimester.

While morning sickness is awful, it does come with a significant silver lining: Nausea tends to signals a healthy pregnancy. Women with nausea have a much lower risk of miscarrying and–as is less widely known–a lower chance of preterm labor.

Morning Sickness and Risk of Miscarriage

For those experiencing nausea, the risk of miscarriage is not just a tiny bit lower, but a huge whopping amount lower than that of women with no nausea.

Woman with nausea have only a third of the chances of a miscarriage as other women. The effect is even stronger among women of “advanced maternal age”–that is, over 35: They have only about a fifth the odds of a miscarriage as other women over 35 with no nausea.

Despite these large effects, if you do not have any nausea, do not panic. A lack of morning sickness does not necessarily imply an impending miscarriage. A lucky 20-30% of pregnant women never experience any morning sickness but give birth to perfectly healthy babies.

Timing also matters: Before 7 weeks, a lack of nausea does not predict miscarriage risk.

Name Calling: Nausea and Vomiting of the Pregnancy (NVP) Versus Morning Sickness

Before we dig into the details, let’s get our terminology straight: Although nausea and vomiting during pregnancy are often referred to as “morning sickness”, most medical professionals prefer the term nausea and vomiting of the pregnancy (NVP). “Morning” sickness, they feel, is misleading.

As many first-time mums-to-be discover to their dismay, nausea often lasts all day long, not just in the morning, . In fact, in one study, less than 2% of women with “morning sickness” had nausea and vomiting only in the morning. Other studies have put the percentage of morning-only suffers at 14%.

The Onset of NVP and Miscarriage Risk

On average, women start to experience NVP 39-40 days after their last menstrual period, around the middle of the 5th week of pregnancy (counting from a woman’s last menstrual period), Symptoms typically begin to ease by around 12 weeks and usually disappear completely by 20 weeks.

That said, 39 days is only the average day of symptom onset. For an unlucky 10% of women, NVP begins much earlier, before they even miss their period. For the 70-80% women who will experience any morning sickness, though, that all day queasy, on-a-winding-road-with-a-bad-hangover feeling will have started by the end of your 8th week of pregnancy, or 7 weeks after conception.

So it’s only in 9th week of pregnancy that a lack of morning sickness predicts higher chances of a miscarriage, according to a prospective study that tracked symptoms of 2407 pregnant women from early in their first trimester.

Line chart shows chances of miscarriage peak in week 8 among those without nausea and vomiting, and remain higher until 13 weeks.

When a women’s nausea begins does not matter. Women who had nausea starting at 4 weeks or at 7 weeks have about the same low chances of a miscarriage.  Similarly, the end of nausea does not appear to make much of a difference. By 12 weeks, the risk of women without nausea drops back down to that of women with nausea.

What Exactly Is Morning Sickness and Why Does It Predict Miscarriage?

So we know that NVP is strongly related to the chances of a healthy, successful pregnancy. But why?

NVP is one of pregnancy’s great mysteries. We don’t know why it occurs. We don’t even know what hormones or other biological changes causes NVP. And we don’t know whether NVP serves a a specific adaptive purpose, as some evolutionary theorists have proposed, or whether it is just an unpleasant side effect of hormonal shifts during early pregnancy.

Rapid rises in hormones like estrogen, progesterone, and human chorionic gonadotropin (HCG) are the prime suspects. So far, though, the evidence for their role is only circumstantial.

Consider HCG, the hormone with the most evidence for a role in NVP. HCG levels in a woman’s blood rises rapidly during the early weeks of pregnancy, doubling every 2 to 3 days. Its concentration peaks sometime between 8-10 weeks of pregnancy.

HCG’s rise corresponds when NVP symptoms are usually at their worst. Conditions which cause high HCG levels like Down’s Syndrome, molar pregnancies, and twin pregnancies are often linked with severe NVP.

That said, HCG levels do not reliably distinguish women with and without NVP, and no one understands why, at a biological level, HCG would induce nausea.

Another potential suspect is pregnant women’s bloodhound-like sense of smell. In a small study of 9 women who had congenital anosmia–they were born with without the ability to smell–only 1 of the 9 suffered from NVP during pregnancy, a rate substantially lower than the usual 4 out 5.

Despite our poor understanding what causes nausea biologically, few researchers believe that a lack of symptoms causes miscarriage.

Why not? For one, treating NVP does not lead to worse pregnancy outcomes. If anything, the opposite is true: Women who take anti-nausea medications have better outcomes, on average, than women who do not take anti-nausea medications–not because treatment itself improves outcomes, but because severe NVP severe usually indicates a healthy placenta.

The Bottom Line

For everyone discretely ducking out of meetings to quietly vomit in the ladies room, there is light at the end of the tunnel. And knowing that your sickness likely means that your pregnancy may provide some comfort. I always clung to that fact as I was clinging to the toilet bowl after dinner.

But please do not think I am trying to minimize the dreadfulness of nearly non-stop nausea.

Nausea and vomiting are more than a simple inconvenience. Women with NVP, even those with so-called “mild” NVP accompanied by little or no vomiting, commonly report decreased productivity at work, taking sick time, strained relationships with their partners, and heightened anxiety and depression.

For around 1 in 100 pregnant women, NVP is life-threatening. Women with especially severe NVP, a condition known as hyperemesis gravidarum, suffer from such severe nausea that they cannot keep food or water down, and require hospitalization. In the U.S. each year, around 50,000 women are hospitalized for severe NVP.

If you are vomiting several times a day or unable to keep fluids down, seek medical help right away. Early treatment may help prevent NVP from becoming dangerously severe.

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:

Chart shows commonly stated chances of miscarriage by pregnancy week. For 1-2 weeks, the chances are 75%. For weeks 3-6 the chances are 10%. For weeks 6-12 the chances are 5%, and for weeks 12-20, the chances drop to 3%.
Commonly reported chances of miscarriage by pregnancy week

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

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?