The placenta starts forming almost immediately after implantation, but it is not fully mature until around the end of the first trimester. Implantation usually happens about 6 to 12 days after fertilization. Cells from the early embryo begin remodeling the lining of the uterus within days of attaching there. The placenta is functionally established and taking over hormone production by roughly 10 to 12 weeks of pregnancy.
Most people picture the placenta as a structure that appears at some point and then sits there. It does not work that way. The placenta is a living organ that grows, changes shape, and changes function across the whole pregnancy. Its early weeks look nothing like its later weeks.
When Does The Placenta Develop and What Are the Stages?
The placenta develops in overlapping phases, not one single event. The earliest steps begin before many people know they are pregnant.
After fertilization, the egg divides into a ball of cells called a blastocyst. By the time it reaches the uterus, it has an outer layer of cells called the trophoblast. This layer is the future placenta. Once the blastocyst attaches to the uterine wall, trophoblast cells begin to invade the tissue beneath them.
Two distinct cell populations form. One is the cytotrophoblast, which divides and supplies new cells. The other is the syncytiotrophoblast, a multinucleated layer that erodes into maternal tissue and later sits in direct contact with maternal blood. The syncytiotrophoblast is the surface where oxygen and nutrients cross from mother to fetus.
By around the third week after fertilization, finger-like projections called villi have formed. These villi are the working units of the placenta. They increase the surface area available for exchange.
Between roughly weeks 6 and 12, the basic architecture is laid down. The placenta is present and functioning, but it is still developing its internal structure. From about week 12 onward, it continues to grow in size and mature in fine structure until late pregnancy.
What Does the Placenta Do in Early Pregnancy?
In the first weeks, the placenta takes over hormone production from the ovary. This handover is one of the most important events of early pregnancy.
After ovulation, the ovary’s corpus luteum produces progesterone to maintain the uterine lining. Early trophoblast cells produce a hormone called human chorionic gonadotropin (hCG), which keeps the corpus luteum working. Over the first trimester, the placenta itself increasingly produces progesterone. By roughly 10 to 12 weeks, the placenta is producing enough progesterone that the corpus luteum is no longer essential.
This is why the timing matters clinically. The placenta is not just a passive filter. It is an endocrine organ, meaning it makes hormones. It produces hCG, progesterone, estrogen, and human placental lactogen, among others.
The placenta also begins exchanging oxygen, nutrients, and waste between mother and fetus. In the earliest weeks, exchange is limited by the immature structure. As villi develop and maternal blood flow into the space around them increases, exchange capacity rises.
How Does the Placenta Change Across the Trimesters?
The placenta is not the same organ at 8 weeks as it is at 38 weeks. Its structure and its job both shift.
First trimester
The placenta is building itself. Villi form and branch. The barrier between maternal and fetal blood is relatively thick compared with later pregnancy, which limits some transfer. Hormone production ramps up. By the end of the first trimester, the placenta has become the main source of progesterone.
Second trimester
The placenta grows steadily in size and surface area. The villi become more complex and better supplied with fetal blood vessels. Exchange capacity increases. On ultrasound, the placenta is usually easy to see and its position is recorded.
Third trimester
Growth slows and the placenta matures. The barrier between maternal and fetal blood becomes thinner, which supports efficient transfer. The placenta produces large amounts of hormones that support fetal growth and prepare the body for labor and breastfeeding.
One point that surprises many people: the placenta does not keep growing until delivery. In many pregnancies its growth rate levels off in late pregnancy, while the fetus continues to grow. Placental function near term is a balance between a structure that has stopped expanding and a fetus with rising demands.
When Can the Placenta Be Seen on Ultrasound?
The placenta can usually be identified on ultrasound by around 8 to 10 weeks of pregnancy, and sometimes a little earlier with high-resolution imaging.
Before that, the early placental tissue is small and blends in with the surrounding decidua, the thickened lining of the uterus. By the end of the first trimester, the placenta is distinct enough that its position can be described, such as anterior, posterior, or fundal.
Early pregnancy ultrasound is more often used to confirm viability, date the pregnancy, and check the number of fetuses than to assess the placenta in detail. Placental position becomes more clinically relevant later, especially in the second and third trimesters, when it can affect delivery planning.
How Big Is the Placenta at Each Stage?
The placenta grows from a microscopic cluster of cells to an organ that, at term, typically weighs roughly one-sixth of the baby’s birth weight. For a typical term baby, that is often in the range of 400 to 600 grams, though normal placentas vary widely.
Exact size at each week is not something you can pin to a single number. Placental size depends on gestational age, fetal size, and individual variation. Ultrasound can estimate placental thickness and appearance, but there is no universal week-by-week size chart that applies to every pregnancy.
What matters clinically is not a specific size at a specific week. It is whether the placenta appears to be functioning normally and whether its position and appearance are consistent with the stage of pregnancy.
What Can Affect How the Placenta Develops?
Placental development is influenced by both maternal and fetal factors. Some of these are well established.
- Maternal blood pressure and blood sugar control can affect placental blood flow and structure.
- Smoking during pregnancy is associated with changes in placental development and function.
- Certain infections in early pregnancy can affect placental tissue.
- Multiple pregnancies, such as twins, involve more placental tissue and sometimes shared placental circulation.
- Some maternal medical conditions are linked to placental complications, which is why prenatal care monitors for them.
The placenta also has a limited ability to adapt. If maternal blood flow is reduced, the placenta may change its structure to try to compensate. These adaptations are not always enough, which is why conditions such as preeclampsia and fetal growth restriction are linked to placental problems.
The evidence here is strongest for associations, not for simple cause-and-effect in every case. Many pregnancies with risk factors have normal placental function, and some placental problems occur in pregnancies with no obvious risk factors.
Does the Placenta Stop Working Before Birth?
No. The placenta keeps working right up to delivery. It does not shut down on a schedule.
What changes near term is that the placenta’s growth has largely stopped while the fetus keeps growing. In some pregnancies, the placenta may not keep up with fetal demands, which is one reason clinicians monitor fetal growth and wellbeing in late pregnancy.
After delivery, the placenta separates from the uterine wall and is delivered as the afterbirth. This typically happens within a short time after the baby is born, though the exact timing varies. Retention of the placenta beyond a certain point is a medical concern that clinicians manage.
What Are the Most Common Myths About Placenta Development?
One common myth is that the placenta “takes over” all at once at 12 weeks. In reality, the handover of hormone production is gradual, and the placenta has been functioning for weeks before that point.
Another myth is that the placenta is fully formed by the end of the first trimester. It is functional by then, but its structure continues to mature throughout pregnancy.
A third myth is that a larger placenta is always better. Placental size alone is not a reliable measure of function. What matters is how well the placenta supports the fetus, which is assessed through fetal growth, blood flow studies, and other clinical measures.
There is also a persistent claim that eating the placenta has proven health benefits. No large, well-controlled human trials have confirmed benefits for placenta consumption, and safety concerns exist. The evidence does not support presenting it as beneficial.
Frequently Asked Questions
When does the placenta fully form?
The placenta is functionally established by about 10 to 12 weeks of pregnancy, but its structure continues to mature throughout pregnancy. It is not fully mature until late pregnancy.
Does the placenta develop at 4 weeks?
Yes. Early placental cells begin forming within days of implantation, which usually occurs about 6 to 12 days after fertilization. By 4 weeks of pregnancy, early placental tissue is already developing.
What happens if the placenta does not develop properly?
Problems with placental development can affect fetal growth and pregnancy outcomes, and clinicians monitor for these through prenatal care. Many pregnancies with early placental concerns still progress normally, but some require closer monitoring.
Can you see the placenta at 6 weeks?
The placenta is usually too small to identify clearly on standard ultrasound at 6 weeks. It is more commonly seen by around 8 to 10 weeks.
