The placenta starts forming before most people know they are pregnant. It begins as early as the first days after implantation, when cells from the fertilized egg attach to the uterine wall and start building the interface between parent and fetus. From there, it grows steadily through the first trimester, expands quickly in the second, and reaches its functional peak late in the third. It is not a static organ. It is a living, changing structure with its own growth curve.
What Is the Placenta and Why Does It Matter?
The placenta is a temporary organ that develops during pregnancy. It attaches to the wall of the uterus and connects to the fetus through the umbilical cord. It is the only organ in the human body that is built from two people — the parent and the fetus.
Its main jobs are nutrient and gas exchange. Oxygen and nutrients from the parent’s blood cross into fetal blood. Carbon dioxide and waste products move the other way. The placenta also produces hormones that sustain the pregnancy, including human chorionic gonadotropin (hCG), progesterone, and estrogen.
The placenta is not a filter in the simple sense. Many substances can cross it. Some medications, alcohol, and certain infections can pass through. That is why safety guidance during pregnancy often focuses on what the parent takes in.
When Does Placenta Grow — A Week-by-Week Timeline
The placenta begins forming at implantation, which usually happens about 6 to 10 days after fertilization. By the time a pregnancy test turns positive, the placenta is already under construction.
Here is how its growth unfolds across the trimesters.
- Weeks 4–8: The placenta is a thin layer of cells. It is not yet fully functional. The yolk sac still helps provide early nutrition to the embryo.
- Weeks 8–12: The placenta takes over hormone production and nutrient exchange. It becomes the primary lifeline. By the end of the first trimester, it is fully formed in structure.
- Weeks 12–20: The placenta grows in surface area to match the growing fetus. It thickens and develops more blood vessels.
- Weeks 20–36: The placenta continues to grow but at a slower pace relative to fetal growth. It reaches its maximum size near the end of the third trimester.
- Weeks 36–40: In many pregnancies, the placenta stops growing or even slightly shrinks. The fetus continues to gain weight. This shift in ratio is normal.
At full term, a typical placenta weighs about 1 pound (roughly 500 grams). It is about 8 to 10 inches across and 1 to 2 inches thick. These are averages, not targets. Normal placentas vary in size and shape.
How Does the Placenta Grow So Quickly?
The placenta grows fast because it is building an entire vascular network from scratch. Early in pregnancy, specialized cells called trophoblasts invade the uterine lining and remodel the parent’s blood vessels. This creates a high-flow, low-resistance system that can deliver enough blood to support fetal growth.
That remodeling is why the placenta has such a rich blood supply. By the end of pregnancy, roughly 20% of the parent’s blood flow goes to the uterus and placenta. The heart has to work harder to pump that volume.
The growth is also driven by hormones and growth factors produced locally. The placenta essentially builds its own support system. This is one reason why conditions that affect blood vessels — such as high blood pressure or diabetes — can affect placental function.
What Can Affect How the Placenta Grows?
Placental growth varies from pregnancy to pregnancy. Some factors are within a person’s control. Many are not.
Things that can influence placental development include:
- Maternal nutrition and overall health before and during pregnancy
- Blood pressure and blood sugar levels
- Smoking, alcohol, and certain drug use
- Infections during pregnancy
- Multiple pregnancy (twins, triplets)
- Genetics and previous pregnancy history
- High altitude living, which can increase placental size
Some of these effects are well established. Smoking, for example, is consistently linked to abnormal placental development and higher risk of complications. Others are less clear. Research on diet and placental size is ongoing, and findings are mixed.
It is also worth knowing that a larger placenta is not automatically better. Both unusually small and unusually large placentas can be associated with complications. Size alone does not determine outcome.
How Do Doctors Check Placental Growth?
There is no routine test that directly measures placental growth in a standard pregnancy. Instead, clinicians assess placental function indirectly.
Ways they check include:
- Ultrasound: Can show placental location, thickness, and structure. It can also detect some abnormalities.
- Fetal growth monitoring: A fetus growing at a normal rate is a sign the placenta is working. Poor fetal growth can signal placental problems.
- Doppler ultrasound: Measures blood flow in the umbilical cord and uterine arteries. Abnormal flow patterns can suggest placental issues.
- Blood tests: Levels of certain hormones and proteins produced by the placenta can give clues about function.
After delivery, the placenta is examined. Weight, shape, and appearance are recorded. This can sometimes explain complications during pregnancy or delivery.
What Happens to the Placenta After Birth?
After the baby is born, the uterus continues to contract. These contractions shear the placenta away from the uterine wall. This usually happens within 5 to 30 minutes after delivery. The placenta is then expelled — a process called the third stage of labor.
If the placenta does not deliver completely, pieces left behind can cause bleeding or infection. Clinicians check the placenta carefully to make sure it is intact. If fragments remain, they may need to be removed.
Some people choose to keep their placenta. Practices like placentophagy — eating the placenta — have gained attention. There is no clinical evidence that eating the placenta provides health benefits. The Centers for Disease Control and Prevention has reported at least one case of a newborn becoming ill after the parent took placenta capsules. The evidence here is limited and the safety concern is real.
When Should You Talk to a Doctor About Placental Concerns?
Most pregnancies have no placental problems. But some symptoms warrant a call to a healthcare provider right away.
Seek medical attention if you experience:
- Vaginal bleeding at any point in pregnancy
- Severe abdominal pain
- Sudden swelling of the face or hands
- Severe headache that does not go away
- Vision changes
- Decreased fetal movement
These can be signs of conditions like placental abruption or preeclampsia. Both need prompt evaluation. Early detection makes a difference.
Routine prenatal care is the best way to monitor placental health. Ultrasound, blood pressure checks, and fetal growth measurements are standard for a reason. They catch problems before they become emergencies.
Frequently Asked Questions
When does the placenta start growing?
The placenta begins forming at implantation, usually 6 to 10 days after fertilization. By the time a pregnancy test is positive, it is already developing.
How big is a full-term placenta?
A typical full-term placenta weighs about 1 pound (500 grams) and measures 8 to 10 inches across. Size varies widely among normal pregnancies.
Does the placenta keep growing until birth?
It grows steadily through the second trimester and reaches its maximum size near the end of the third trimester. In many pregnancies, it stops growing or slightly shrinks in the final weeks.
Can you measure placental growth during pregnancy?
There is no routine direct measurement of placental growth. Doctors assess placental function indirectly through ultrasound, fetal growth monitoring, and blood flow studies.
