The placenta gradually takes over hormone production between about 8 and 12 weeks of pregnancy, and morning sickness typically improves around weeks 12 to 14. These two events are closely linked, but the relationship is more nuanced than a simple on/off switch. Understanding what’s actually happening with your hormones during this transition helps explain why nausea doesn’t vanish overnight.
When the Placenta Takes Over
For the first several weeks of pregnancy, a temporary structure called the corpus luteum (the remnant of the egg’s follicle on your ovary) produces the progesterone and estrogen needed to sustain the pregnancy. Starting around week 8, the placenta begins producing these hormones on its own. By roughly week 10 to 12, placental production becomes dominant, and the corpus luteum degenerates. This handoff is sometimes called the “luteal-placental shift.”
The shift doesn’t happen in a single moment. It’s a gradual transition where the placenta ramps up while the corpus luteum winds down. One key hormone in this process, hCG (the hormone detected by pregnancy tests), rises rapidly in early pregnancy, almost doubling every three days, and peaks around week 10. After that peak, hCG gradually declines for the rest of the pregnancy. That decline signals that the placenta has taken the reins.
Why Morning Sickness Peaks When It Does
Morning sickness typically starts around week 6, feels worst between weeks 8 and 10, and begins improving by weeks 12 to 14. That timeline closely mirrors the rise and fall of hCG, which is why the two have long been assumed to be connected. But a landmark 2023 study published in Nature identified a different culprit: a hormone called GDF15.
The vast majority of GDF15 circulating in a pregnant person’s blood comes from the fetus and placenta. This hormone triggers nausea and vomiting by acting on a specific part of the brainstem. What makes the picture interesting is that severity depends not just on how much GDF15 is being produced, but on how sensitive your body is to it. Women who had low levels of GDF15 before pregnancy tend to react more strongly when levels spike. Women with conditions that keep GDF15 chronically elevated (like beta-thalassemia) report very little pregnancy nausea, because their bodies have essentially become desensitized to it.
This desensitization effect works both ways. As pregnancy progresses and your body is exposed to steadily rising GDF15 for weeks, your sensitivity gradually decreases. That’s one reason morning sickness eases: not because the hormone disappears, but because your body adjusts to it.
The Typical Morning Sickness Timeline
Most women start noticing nausea before 9 weeks. Symptoms peak somewhere between weeks 10 and 16, then gradually fade. For the majority, nausea improves significantly by week 13 or 14, right around the start of the second trimester. By week 20, the vast majority of women are symptom-free.
About 10% of women continue experiencing nausea beyond 22 weeks. This is frustrating but not necessarily a sign that something is wrong. Every pregnancy produces a different hormonal profile, and individual sensitivity to those hormones varies widely.
Why the Placental Shift Doesn’t Instantly Fix Nausea
It’s tempting to think of the placental takeover as a clean cutoff: the placenta takes over, hormones stabilize, nausea stops. In reality, the placenta doesn’t reduce the hormones causing nausea. It actually produces GDF15 itself. What changes is the overall hormonal landscape. HCG starts declining after week 10, progesterone production stabilizes under placental control, and your body has had weeks to adapt to the hormones circulating in your system.
Progesterone levels continue rising throughout pregnancy. Once the placenta is fully in charge (around week 10), progesterone climbs from roughly 8 to 48 ng/mL in the second trimester to as high as 342 ng/mL by the third trimester. Despite those rising levels, nausea doesn’t return for most women, further supporting the idea that desensitization plays a major role in symptom relief.
Severe Nausea Follows a Different Timeline
Hyperemesis gravidarum, the severe form of pregnancy nausea involving persistent vomiting, weight loss, and dehydration, follows a longer course. Symptoms typically begin about two weeks after the first missed period, peak around week 12, and may not resolve until week 20 or later. This condition affects a small percentage of pregnancies but can require medical treatment to manage fluid and nutrient loss.
If your nausea is so severe that you can’t keep fluids down, are losing weight, or feel dizzy and weak, that pattern goes beyond typical morning sickness. The placental transition won’t resolve hyperemesis on the same schedule as ordinary nausea.
What You Can Realistically Expect
If you’re in the thick of morning sickness at weeks 8 to 10, the worst is likely happening right now. Most women notice a meaningful improvement between weeks 12 and 14, with the placental shift and hormonal adaptation working together to ease symptoms. But “improvement” often means a gradual fade rather than a sudden stop. You might have a few good days followed by a rough one, with the good days slowly becoming more frequent.
Some women feel dramatically better the moment they cross into the second trimester. Others find nausea lingers at a low level until weeks 16 to 20. Both patterns are normal. The wide range exists because the severity of morning sickness depends on a combination of how much GDF15 your fetus produces, how sensitive your body was to that hormone before pregnancy, and how quickly you desensitize as exposure continues.