The placenta begins forming within days of conception, built from cells that originally belong to the embryo itself. It starts as a thin layer of specialized cells on the outside of the fertilized egg and, over the course of several weeks, develops into a complex organ that weighs about 1.5 pounds at full term. The process involves both the embryo and the mother’s uterine lining working together in a tightly coordinated sequence.
Implantation: The First Step
Around day 5 after fertilization, the embryo exists as a hollow ball of cells called a blastocyst. It hatches from its protective outer shell and, by day 6 or 7, presses against the uterine wall and begins to attach. The cells on the outer surface of the blastocyst are called trophoblast cells, and they are the raw material from which the placenta will be built.
Just before the blastocyst makes contact with the uterine lining, these trophoblast cells split into two distinct layers. The inner layer acts as a reserve of progenitor cells that will keep dividing throughout pregnancy. The outer layer fuses into a single continuous sheet of shared cytoplasm containing millions of nuclei. This fused outer layer is the part that actively burrows into the uterine wall, anchoring the embryo in place.
How the Uterine Lining Responds
The mother’s body doesn’t passively receive the embryo. In response to rising progesterone levels and signals from the embryo itself, the cells lining the uterus undergo a transformation called decidualization. The normally thin, fibrous stromal cells swell and change shape, becoming specialized decidual cells. This process requires at least 8 to 10 days of hormone stimulation and begins even before the embryo arrives, during the second half of the menstrual cycle.
The transformed tissue directly beneath the implantation site becomes the decidua basalis, which forms the maternal side of the placenta. Decidualization eventually spreads across the rest of the uterus as well, but the decidua basalis is the critical zone where maternal and fetal tissues will interlock.
Building the Villi: Weeks 2 Through 4
The placenta’s working units are tiny finger-like projections called chorionic villi, and they develop in three stages over the first several weeks of pregnancy.
- Primary villi (week 2): The trophoblast layers push outward into the maternal tissue, forming simple finger-like extensions. At this point, the projections are solid columns of trophoblast cells with no internal structure.
- Secondary villi (week 3): A core of loose connective tissue grows into each projection, giving the villi an internal scaffold. These villi initially cover the entire surface of the embryonic sac.
- Tertiary villi (week 4): Blood vessels form within the connective tissue core, creating a network of tiny capillaries that connect to the embryo’s developing circulatory system through the umbilical cord. This is the stage where the villi become functional, capable of exchanging oxygen and nutrients between two separate bloodstreams.
Over time, villi on the side of the sac facing away from the uterine wall degenerate, while those anchored in the decidua basalis grow larger and branch extensively. This concentrates the placenta into a single disc-shaped organ.
Remodeling the Mother’s Blood Vessels
One of the most remarkable parts of placenta formation happens inside the walls of the mother’s uterine arteries. Trophoblast cells from the placenta migrate deep into these spiral-shaped arteries and essentially dismantle them from the inside. They trigger the smooth muscle cells in the artery walls to detach and die off through programmed cell death, and they break down the structural scaffolding that keeps the vessels narrow.
The result is that the small, tightly regulated arteries are converted into wide, low-resistance channels that deliver large volumes of maternal blood directly to the placenta. Without this remodeling, the placenta cannot receive enough blood flow to support a growing fetus. When the process goes wrong, it contributes to complications like preeclampsia and fetal growth restriction.
The Placental Barrier
Despite the close proximity of maternal and fetal blood in the villi, the two bloodstreams never actually mix. They are separated by a thin barrier made up of the fused outer trophoblast layer on one side, a thin strip of connective tissue in the middle, and the walls of the fetal capillaries on the other side. Oxygen, carbon dioxide, nutrients, and waste products diffuse across this barrier, while larger threats like many pathogens and maternal immune cells are blocked.
The outer trophoblast layer also releases small fragments, proteins, and even fragments of fetal DNA into the mother’s bloodstream. These molecules help modulate the mother’s immune system so it tolerates the genetically foreign pregnancy. This same fetal DNA circulating in maternal blood is what makes noninvasive prenatal screening possible through a simple blood draw.
Taking Over Hormone Production
For the first several weeks of pregnancy, a temporary structure in the ovary called the corpus luteum produces the progesterone needed to maintain the uterine lining. Starting around the 7th week of gestation, the placenta begins producing substantial amounts of progesterone on its own. This transition, sometimes called the luteoplacental shift, is a critical handoff. Once complete, the placenta becomes the pregnancy’s primary hormone factory.
Beyond progesterone, the placenta produces estrogen, human chorionic gonadotropin (the hormone detected by pregnancy tests), placental lactogen (which adjusts the mother’s metabolism to prioritize fetal nutrition), and a dedicated form of growth hormone. It also secretes growth factors that promote blood vessel development and signaling molecules that fine-tune the mother’s immune response throughout pregnancy.
Growth and Maturation
The placenta continues to grow and add functional tissue until roughly 36 weeks of gestation. During this first stage, the villi branch into increasingly fine projections, the barrier between maternal and fetal blood thins, and the total surface area for exchange expands dramatically. After 36 weeks, a maturation phase begins: the fetus keeps growing rapidly, but the placenta itself stops adding new tissue. Instead, existing structures become more efficient at transport.
At full term, the placenta weighs about 1.5 pounds (roughly 0.7 kilograms) and is shaped like a flat disc, typically 15 to 20 centimeters across. The fetal side is smooth and covered by membranes, with the umbilical cord inserting near the center. The maternal side is rough and divided into 15 to 20 lobes, each containing a cluster of villi bathed in maternal blood. It is delivered shortly after the baby during the third stage of labor.