When Does The Placenta Take Over During Pregnancy?

The placenta starts forming right after conception, but it does not fully take over hormone production until around week 8 to week 10 of pregnancy. Before that point, a structure called the corpus luteum in the ovary produces the hormones needed to sustain the pregnancy. By the end of the first trimester, the placenta has matured enough to become the main source of progesterone and estrogen, which is why this transition is often called the “luteal-placental shift.”

What Does the Placenta Actually Do?

The placenta is a temporary organ that grows inside the uterus during pregnancy. It attaches to the uterine wall and connects to the baby through the umbilical cord. It acts as the lifeline between mother and baby.

Its main jobs include delivering oxygen and nutrients to the growing baby, removing waste products from the baby’s blood, and producing hormones that maintain the pregnancy. It also allows some antibodies to pass from mother to baby, which gives the newborn some immune protection after birth.

The placenta is the only organ that two different people share. The mother’s blood and the baby’s blood never directly mix. Instead, they pass materials across a thin membrane barrier. This barrier allows oxygen, glucose, amino acids, and other nutrients to cross into the baby’s circulation while keeping most larger molecules and blood cells separate.

When Does the Placenta Take Over Hormone Production?

The corpus luteum is a temporary endocrine gland that forms in the ovary after ovulation. After an egg is fertilized, the corpus luteum secretes progesterone and estrogen. Progesterone is critical in early pregnancy because it keeps the uterine lining thick and prevents the uterus from contracting.

Around week 8 of pregnancy, the placenta begins producing significant amounts of progesterone. By week 10, the placenta has typically become the dominant source of this hormone. This handover is gradual rather than sudden. For a week or two, both the corpus luteum and the placenta contribute to hormone levels.

If the placenta does not take over properly, the pregnancy cannot continue. The corpus luteum begins to break down if it is not supported, and falling progesterone levels would trigger miscarriage. This is why the transition between weeks 8 and 10 is a critical window in early pregnancy.

Some clinicians measure progesterone levels in early pregnancy to monitor this transition. However, routine progesterone screening is not standard for all pregnancies. It is typically reserved for women with a history of recurrent miscarriage or for pregnancies conceived with fertility treatment.

Why the Corpus Luteum Matters Before Week 8

In the first weeks after conception, the embryo is tiny and has no way to sustain itself. It relies entirely on the corpus luteum for hormonal support. The corpus luteum produces progesterone in response to human chorionic gonadotropin (hCG), the hormone detected by pregnancy tests.

As the embryo implants and grows, it secretes increasing amounts of hCG. This hCG signal tells the corpus luteum to keep producing progesterone. Without this signal, the corpus luteum would degenerate, progesterone would fall, and the uterine lining would shed — which is what happens during a normal menstrual cycle.

This is why hCG levels rise so rapidly in early pregnancy. The embryo is essentially sending a continuous chemical message to the ovary: keep making progesterone. Once the placenta is mature enough to make its own progesterone, the dependence on hCG and the corpus luteum decreases.

By week 12, the corpus luteum has usually regressed significantly. The placenta is now fully responsible for maintaining the pregnancy. This is also roughly when the risk of miscarriage drops substantially, which aligns with the successful completion of this hormonal transition.

Does the Placenta Take Over Everything at Once?

No. The placenta takes over different functions at different times. Hormone production is the main event around weeks 8 to 10, but nutrient and oxygen exchange begins earlier and continues to develop throughout the pregnancy.

Blood flow to the placenta increases steadily. Early in pregnancy, the placenta is small and the blood supply is limited. As the pregnancy progresses, the placenta grows and the network of blood vessels within it expands. By the third trimester, the placenta is a large, highly vascular organ that can transfer substantial amounts of oxygen and nutrients.

Some placental functions peak at different times. For example, the transfer of antibodies from mother to baby mostly happens in the third trimester. This is why babies born prematurely have lower antibody levels than full-term babies.

The placenta also produces other hormones beyond progesterone and estrogen. These include human placental lactogen (hPL), which affects the mother’s metabolism, and relaxin, which helps soften the cervix and pelvic ligaments. These hormones appear at different times and in different amounts as the pregnancy advances.

What Happens if the Placenta Does Not Take Over Properly?

In some cases, the placenta does not develop or function as expected. This can lead to complications that range from mild to severe.

Placental insufficiency is a condition where the placenta does not deliver enough oxygen and nutrients to the baby. This can cause slow fetal growth, which doctors call intrauterine growth restriction (IUGR). It can also lead to low birth weight and other complications.

Placenta previa occurs when the placenta covers the cervix. This can cause bleeding, especially in the third trimester, and often requires delivery by cesarean section.

Placental abruption is a condition where the placenta separates from the uterine wall before delivery. This is a medical emergency because it can cut off the baby’s oxygen supply and cause heavy bleeding in the mother.

These conditions are not caused by the timing of the hormone handover. They are usually related to how the placenta implants, grows, or attaches. Most placental problems are detected through routine ultrasound scans, which is why prenatal care includes regular imaging in the second and third trimesters.

It is also worth noting that some research suggests a link between certain pregnancy complications and placental dysfunction. However, the exact causes are often not fully understood. In many cases, a placental problem has no clear explanation.

Can You Feel When the Placenta Takes Over?

No. The luteal-placental shift happens without any noticeable physical sensation. You cannot feel the placenta begin to produce hormones.

Some women notice that early pregnancy symptoms change around this time. For example, nausea and fatigue may start to ease toward the end of the first trimester. This is partly because hormone levels stabilize as the placenta takes over. However, this is not a reliable sign that the transition has occurred, because symptoms vary widely from woman to woman and from pregnancy to pregnancy.

Some women have no symptoms at all in early pregnancy and go on to have perfectly healthy pregnancies. Others have severe nausea well beyond the first trimester. Symptom changes are not a diagnostic tool for placental function.

The only way to confirm that the placenta is functioning properly is through medical testing. Ultrasound scans can measure fetal growth, amniotic fluid levels, and blood flow in the umbilical cord. Blood tests can measure hormone levels. But these tests are not done routinely in every pregnancy, and most healthy pregnancies do not need them.

When Does the Placenta Take Over in IVF Pregnancies?

In pregnancies conceived through in vitro fertilization (IVF), the timing of the luteal-placental shift is essentially the same as in natural conception. The placenta still becomes the main hormone source around weeks 8 to 10 of pregnancy.

However, IVF pregnancies often involve extra hormonal support. Many IVF patients take progesterone supplements, either as injections, vaginal suppositories, or oral medication, for the first 8 to 12 weeks of pregnancy. This is because the corpus luteum may not function as strongly after the egg retrieval process, which can disrupt the normal ovarian cycle.

The progesterone supplementation is usually continued until the placenta has clearly taken over. Doctors typically wean patients off the supplements between weeks 10 and 12, once ultrasound and blood tests confirm that the pregnancy is progressing normally.

What Is the Placenta’s Role After the Baby Is Born?

The placenta does not stop being important at delivery. After the baby is born, the placenta is expelled from the uterus in what is called the third stage of labor. This usually happens within 5 to 30 minutes after birth.

Some families choose to keep the placenta and have it encapsulated, buried, or used in other cultural practices. Placenta encapsulation involves drying and grinding the placenta into pills. Some claim this helps with postpartum mood, energy, and milk supply.

The evidence for these benefits is limited. No large clinical trials have confirmed that eating the placenta provides measurable health benefits. Some research suggests that placental pills may not contain meaningful amounts of hormones after the drying process. There is also a small risk of infection if the placenta is not handled and stored properly. The CDC has issued warnings about placenta capsule preparation due to potential bacterial contamination. If you are considering placenta encapsulation, discuss it with your healthcare provider first.