The placenta gradually takes over hormone production between weeks 7 and 10 of pregnancy, with the transition completing around week 10. This shift doesn’t happen overnight. It’s a gradual handoff from a temporary structure in your ovary called the corpus luteum to the placenta itself, and it unfolds over several weeks during the first trimester.
What the Placenta Is Taking Over From
After ovulation, the follicle that released your egg transforms into a small hormone-producing gland called the corpus luteum. Its main job is pumping out progesterone, the hormone that keeps the uterine lining thick and stable enough to support a new pregnancy. Without adequate progesterone in those early weeks, the lining would shed and the pregnancy couldn’t continue.
The corpus luteum is only a temporary solution. It was never designed to sustain a pregnancy for nine months. By around week 5, the developing placenta begins producing its own progesterone, starting the process of making the corpus luteum unnecessary. The corpus luteum stays relatively constant in size through weeks 8 to 9, then begins shrinking noticeably from week 10 onward as the placenta assumes full responsibility.
The Luteal-Placental Shift: Weeks 7 Through 10
The formal name for this handoff is the “luteal-placental shift,” and it occurs between weeks 7 and 9, with some overlap extending to week 10. During this window, both the corpus luteum and the placenta are producing progesterone simultaneously. Think of it like two workers sharing a shift before one clocks out. The placenta ramps up production while the corpus luteum gradually winds down.
In normal pregnancies, median progesterone levels rise steadily throughout the first trimester, climbing from roughly 57.5 nmol/L at week 5 to about 80.8 nmol/L by week 13. That smooth upward trend reflects the placenta successfully picking up where the corpus luteum left off. In pregnancies complicated by threatened miscarriage, those levels start lower (around 41.7 nmol/L at week 5) but typically catch up to near-normal levels by week 13, suggesting the placental transition may simply take longer in some cases.
What’s Happening Inside the Placenta
The placenta doesn’t just flip a switch and start working. Its ability to take over depends on a complex process of building blood vessel connections to your uterus. During the first trimester, specialized cells from the placenta (called trophoblast cells) invade the walls of the spiral arteries in your uterus, remodeling them into wider, lower-pressure vessels that can deliver a steady blood supply.
Around week 7, these trophoblast cells actually plug the spiral arteries, temporarily restricting the flow of oxygen-rich blood to the placenta. This sounds counterintuitive, but the low-oxygen environment protects the developing embryo from oxidative stress during a critical period of organ formation. As the plugs gradually dissolve later in the first trimester, full maternal blood flow opens up to the placenta, and it can function at full capacity. Problems with this process, like plugs dissolving too early, can cause oxidative damage and are linked to pregnancy complications like preeclampsia.
Beyond the spiral arteries, the deeper radial and arcuate arteries in the uterus also undergo significant remodeling to support the growing demands of pregnancy.
Why This Transition Matters for Symptoms
Many people notice their worst first-trimester symptoms start easing up right around the time the placental transition completes, and that’s not a coincidence. Morning sickness correlates closely with levels of hCG (human chorionic gonadotropin), the hormone your body produces in large quantities to keep the corpus luteum alive and producing progesterone. Both hCG levels and nausea peak between weeks 12 and 14.
Once the placenta is fully in charge of progesterone production, your body no longer needs to produce as much hCG to sustain the corpus luteum. As hCG levels plateau and then decline, nausea typically improves. Most people see symptoms peak between weeks 10 and 16, with resolution by week 20. So when people say “the second trimester gets easier,” the placental takeover is a big part of why.
That said, the timeline varies. Some people feel better by week 10, others not until well past week 14. The hormonal shift is gradual, and individual sensitivity to those hormones differs widely.
When the Transition Doesn’t Go Smoothly
In some pregnancies, the corpus luteum doesn’t produce enough progesterone in the first place, a condition called luteal phase deficiency. If the corpus luteum falls short before the placenta is ready to take over, progesterone levels can dip during the transition window, raising the risk of miscarriage.
For people with a history of recurrent miscarriage or signs of luteal phase problems (like a consistently short luteal phase or unexplained spotting between periods), progesterone supplementation is sometimes used to bridge the gap. This typically involves vaginal progesterone suppositories started after a positive pregnancy test and continued through the first trimester until the placenta is reliably producing enough on its own. Research has shown that pregnancies can even be sustained with progesterone supplementation after surgical removal of the corpus luteum, as long as the surgery happens before 9 weeks, confirming that the placenta can eventually handle the job independently.
IVF pregnancies represent a common scenario where this matters. Since the egg retrieval process can disrupt normal corpus luteum function, progesterone support is standard in IVF cycles and is typically continued until around weeks 10 to 12, when the placenta has clearly taken over.
What You Can Expect Week by Week
- Week 5: The placenta begins producing small amounts of progesterone. The corpus luteum is still doing the heavy lifting.
- Weeks 7 to 9: The active handoff period. Both structures are producing progesterone, with the placenta steadily increasing its share.
- Week 10: The corpus luteum begins visibly shrinking. The placenta is now the primary source of progesterone.
- Weeks 12 to 14: hCG peaks and starts declining. The placenta is fully in control, and first-trimester symptoms often begin improving.
If you’re on prescribed progesterone supplements, your provider will typically taper them off somewhere in the 10 to 12 week range, timed to when the placenta should be self-sufficient. A drop in symptoms around this time is normal and expected, not a sign that something has gone wrong.