The placenta begins forming almost immediately after implantation, around week 2 of pregnancy, but it doesn’t become a fully functional organ until roughly weeks 10 to 12. That wide window exists because placental development isn’t a single event. It’s a weeks-long construction project where cells invade, blood vessels grow, and the organ gradually takes over the job of feeding and protecting the fetus.
How the Placenta Starts: Weeks 2 Through 4
After a fertilized egg implants in the uterine wall (around 6 to 10 days after conception), the outer layer of cells surrounding the embryo begins burrowing into the uterine lining. These cells are the earliest building blocks of the placenta. By week 2, they form tiny finger-like projections that reach into the mother’s tissue. At this stage, the projections are simple structures with no blood vessels of their own.
By week 3, a layer of connective tissue grows into these projections, giving them more structural support. Then at week 4, something critical happens: blood vessels develop inside the projections and connect to the blood vessels forming in the embryo’s umbilical cord. This creates the first version of the network that will eventually carry oxygen and nutrients between mother and baby. Even so, the placenta is far from finished.
Why the First Trimester Runs on a Backup System
For the first several weeks, the embryo doesn’t rely on the placenta for nutrition. Instead, it’s sustained by the yolk sac, a temporary structure that provides nutrients until the placenta is ready. The ovary also handles hormone production during this period through a structure called the corpus luteum, which pumps out progesterone to maintain the pregnancy.
The handoff from ovary to placenta for hormone production, sometimes called the luteal-placental shift, happens surprisingly early. Research published in Fertility and Sterility found that by 7 weeks of gestation, about 80% of pregnant women showed progesterone levels indicating the placenta had already taken over this job. This transition is one reason the risk of miscarriage drops significantly after the first trimester: once the placenta is producing its own hormones, the pregnancy is on more stable footing.
When Maternal Blood Reaches the Placenta
One of the most important milestones in placental development is when the mother’s blood actually starts flowing through it. This happens later than most people assume. In the earliest weeks, cellular plugs physically block the mother’s blood from reaching the placental surface. This is thought to protect the developing embryo from the full force of oxygenated blood, which could generate harmful free radicals during a sensitive period of organ formation.
These plugs begin breaking down gradually starting around week 6. Some maternal blood cells can reach the placenta as early as weeks 7 to 8, and Doppler ultrasound has detected blood flow in the placental region at week 7 in some pregnancies. But the plugs don’t fully disperse until around weeks 10 to 12, which is when true, robust blood flow to the placenta is established. This is the point at which the placenta takes over as the primary system for gas exchange, delivering oxygen and removing carbon dioxide from the fetal blood.
Remodeling the Mother’s Blood Vessels
To supply the growing fetus, the placenta doesn’t just passively receive blood. It actively remodels the mother’s blood vessels to increase flow. Specialized placental cells invade the walls of the spiral arteries in the uterus, transforming them from narrow, muscular vessels into wide, low-resistance channels that can deliver large volumes of blood.
This remodeling happens in two waves. The first occurs between weeks 8 and 10, affecting the portions of the arteries closest to the uterine surface. The second wave, between weeks 16 and 18, reaches deeper into the uterine muscle. When this process goes wrong, it can restrict blood flow to the placenta and contribute to complications like preeclampsia or fetal growth restriction later in pregnancy.
When the Placenta Is Fully Functional
By the end of the first trimester, around week 12 to 13, the placenta is handling all the major jobs it will carry out for the rest of pregnancy: supplying oxygen and nutrients, removing waste, producing hormones like progesterone and estrogen, and acting as an immune barrier between mother and baby. The yolk sac is no longer needed and has largely broken down by this point.
That said, the placenta continues growing and maturing well beyond the first trimester. It increases in size and weight throughout pregnancy, and its internal structure becomes more efficient at exchange as the blood vessel network branches further. By full term, the placenta weighs roughly 500 grams (just over a pound) and processes about 600 milliliters of maternal blood per minute.
When You Can See It on Ultrasound
On a standard ultrasound, the placenta becomes visible surprisingly early. Specialized imaging techniques like Doppler ultrasound have identified placental structures and blood flow patterns as early as 8 to 9 weeks. At a typical first-trimester ultrasound, performed around weeks 11 to 13, the placenta is clearly visible, and your provider can note its general location. Placental position is monitored more closely at the mid-pregnancy anatomy scan around week 20, when conditions like placenta previa (where the placenta covers the cervix) can be identified. Many placentas that appear low-lying at 12 weeks will have shifted to a normal position by that later scan as the uterus expands.