Once you’ve got your positive pregnancy test you may waiting impatiently for morning sickness to start. An estimated 70 – 80 percent of pregnant women are hit with nausea during the first trimester, and 50 percent experiencing vomiting, both of which can happen at any time of day, not just the morning.
The most common time for morning sickness to start is in the middle of the fifth week of pregnancy (a week and a half after your missed period). However, this is not a rule and the experiences of individual women vary considerably.
For around 13 percent of women, morning sickness can start before they even miss their period, which can be one of the earliest signs of pregnancy they experience. For 90 percent of women who experience morning sickness, it starts by the eight week of pregnancy.
How long does morning sickness last?
The most common pattern is for symptoms to rise sharply beginning in week six, then tail off gradually after week 10. Weeks seven to nine are typically when morning sickness symptoms are at their peak. While morning sickness tends to start suddenly, for most women it tapers off gradually. And for an unlucky 10 percent of pregnant women, morning sickness actually gets worse after week nine.
Although feeling nauseous is no fun, the good news is that it signals a healthy pregnancy. Women with nausea have one third the chance of miscarrying compared to those with no nausea1, as well as a lower chance of preterm labor2.
Is it bad if I don’t have morning sickness?
While women who experience morning sickness have a lower chance of miscarrying, a lack of morning sickness certainly does not mean your pregnancy is doomed! About 20 – 30 percent of women have no morning sickness at all and go on to deliver perfectly healthy babies.
Your likelihood of experiencing morning sickness is also determined in part by your previous pregnancies (morning sickness tends to be more severe with each subsequent pregnancy3) and your ethnic background (white women are more prone to nausea than Black or Asian women4, and Black women are more likely to experience nausea that starts after the first trimester5).
And if you’re early in your pregnancy and worried because you don’t feel nauseous yet, chances are the nausea will hit you in the next few weeks. It’s only in the eighth week of pregnancy that a lack of morning sickness predicts a higher chance of miscarriage, according to a prospective study that tracked 2,407 pregnant women from early in their pregnancies6. According to the study, as long as symptoms started by the eighth week, it didn’t make a difference in the miscarriage rate whether they started early or late. And once the first trimester was over, nausea (or lack thereof) had no bearing on the chances of pregnancy loss.
What causes morning sickness anyway?
No one really knows. Scientists debate whether it serves some kind of evolutionary purpose—perhaps reducing women’s likelihood of consuming spoiled food during pregnancy—or whether it’s simply an unpleasant side effect of increasing hormone levels.
It seems that levels of the hormone hCG play some role in morning sickness; hCG increases exponentially during the first few weeks of pregnancy and peaks between weeks 8 – 10, which coincides with the time when morning sickness is typically at its most severe. And conditions which cause higher than average levels of hCG, like Down’s Syndrome, molar pregnancies, and twins often cause more severe morning sickness.
However, there is a lot of variation in hCG levels in healthy pregnancies, and hCG levels do not reliably predict who gets morning sickness, who doesn’t, and how severe it is.
While no one understands exactly how, biologically, hCG might contribute to morning sickness, one thing that seems clear is that lack of symptoms does not cause a miscarriage. How do we know? Well, for one thing, treating morning sickness does not lead to worse pregnancy outcomes. Actually, the opposite is true: women who are treat their morning sickness tend to have better outcomes than those who do not—not because treatment improves outcomes, but because if morning sickness is severe enough to require treatment, it usually signals a healthy pregnancy.