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.
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.
