The placenta begins producing its own hormones around week 5 of pregnancy, but the full transition happens between weeks 7 and 9. Before that point, a temporary structure on the ovary called the corpus luteum is responsible for producing the progesterone that sustains the pregnancy. By the end of the first trimester (around 12 weeks), the placenta has fully taken over.
The Luteal-Placental Shift, Explained
After ovulation, the follicle that released the egg transforms into the corpus luteum, a small hormone-producing gland on the ovary. Its main job is pumping out progesterone, the hormone that thickens the uterine lining and keeps the pregnancy viable in its earliest days. A hormone called hCG, produced by the developing embryo, signals the corpus luteum to keep working rather than breaking down as it would in a normal menstrual cycle.
Around week 5, the placenta starts producing its own progesterone. For the next few weeks, both the corpus luteum and the placenta contribute progesterone simultaneously. This overlap period, sometimes called the luteal-placental shift, runs roughly from weeks 6 through 8. By week 9, the placenta is typically producing enough progesterone on its own to sustain the pregnancy without the corpus luteum. The corpus luteum itself begins shrinking noticeably from about week 10 onward.
The Progesterone Dip at Week 7
During this handoff, progesterone levels don’t climb in a straight line. They actually dip. A large study tracking progesterone through the first trimester found that levels start declining after week 5, hitting their lowest point at week 7 before rising again through weeks 8 and 9. This temporary drop corresponds to the window when the corpus luteum is winding down but the placenta hasn’t yet reached full capacity.
This is a normal part of early pregnancy, and in most cases the body navigates it without issue. But it helps explain why weeks 6 through 8 can be a vulnerable period. Research on pregnancies where the corpus luteum was surgically removed has shown a clear dividing line: removal before week 7 led to an immediate drop in progesterone and miscarriage, while removal after week 9 did not threaten the pregnancy. That finding is one of the clearest pieces of evidence pinpointing when the placenta becomes self-sufficient.
Why This Timeline Matters for IVF and Progesterone Support
If you conceived through IVF or are taking supplemental progesterone for another reason, you’ve likely wondered when it’s safe to stop. The luteal-placental shift is the biological basis for that decision. Many fertility clinics discontinue progesterone support somewhere between 7 and 12 weeks, though practices vary. Some studies have found that stopping as early as the first positive pregnancy test doesn’t affect pregnancy outcomes in certain patients, while other clinicians prefer to continue until 9 or even 12 weeks as a precaution.
In rare cases, the placenta doesn’t fully take over progesterone production on schedule. This is sometimes called a luteal phase defect, and it can contribute to recurrent first-trimester or even second-trimester pregnancy loss. In these situations, progesterone supplementation may need to continue well beyond the typical stopping point. One documented case involved a patient whose body produced essentially no progesterone on its own throughout pregnancy, requiring supplementation the entire time.
What hCG Levels Tell You
hCG, the hormone detected by pregnancy tests, plays a direct role in this transition. In the first 4 to 6 weeks, hCG keeps the corpus luteum alive and producing progesterone. As the placenta matures and takes over progesterone production, the need for hCG to stimulate the corpus luteum decreases. hCG levels typically peak between 8 and 10 weeks, reaching concentrations of 100,000 to 200,000 mIU/mL, then gradually decline through the second trimester. That peak roughly coincides with the completion of the hormonal handoff.
How Doctors Check Placental Function
There’s no single test that announces “the placenta has taken over.” Instead, doctors rely on a combination of signals. Progesterone blood draws can track whether levels are rising appropriately after the week-7 dip. Ultrasound with Doppler imaging can evaluate blood flow through the uterine arteries, which reflects how well the placenta’s blood supply is developing. High resistance in those arteries or abnormal waveform patterns can signal a poorly developed placental circulation, which may lead to complications later in pregnancy.
For most pregnancies, though, these specialized assessments aren’t necessary. The standard first-trimester ultrasound around weeks 11 to 13 confirms that the pregnancy is progressing normally, and by that point the placenta is firmly in control of hormone production. The corpus luteum has already begun its quiet regression, its job done.