The placenta develops in the wall of the uterus, growing from the spot where a fertilized egg first embeds into the uterine lining. It forms from a combination of fetal and maternal tissues: the outer cells of the embryo burrow into the endometrium (the inner lining of the uterus), and together these tissues build the organ that will nourish the pregnancy for the next nine months. By the time of delivery, the placenta typically weighs around 500 grams and measures roughly 18.5 centimeters across.
How the Placenta Begins to Form
Placental development starts at implantation. About 2 to 4 days after a ball of dividing cells (called a morula) enters the uterine cavity, it becomes a blastocyst and begins attaching to the uterine wall. This happens in three stages. First, the blastocyst drifts into contact with the lining. Then specialized outer cells latch onto the surface using adhesion molecules that work a bit like molecular Velcro. Finally, those outer cells push through the surface layer and invade deeper into the tissue beneath.
Those invading outer cells are called trophoblasts, and they are the earliest building blocks of the placenta. Once embedded, they begin forming finger-like projections called villi that reach into the uterine lining. These villi eventually become densely branched structures where nutrients and oxygen pass between mother and baby, without the two bloodstreams ever directly mixing.
Where in the Uterus It Attaches
The placenta can attach to several different spots on the uterine wall, and the exact location depends on where the embryo happens to implant. In one study of placental positions, 46% of placentas were at the fundus (the top of the uterus), 28% on the anterior wall (the front, closer to the abdomen), and 26% on the posterior wall (the back, closer to the spine). All three positions are normal.
The one position that causes problems is when the placenta grows low in the uterus, partially or fully covering the cervix. This is called placenta previa, and it occurs in roughly 5 out of every 1,000 pregnancies worldwide, though rates vary by population. Placenta previa can cause bleeding during pregnancy and typically requires delivery by cesarean section. Many placentas that appear low-lying on early ultrasounds migrate upward as the uterus expands, so a low position at 20 weeks doesn’t always mean it will remain there.
Maternal and Fetal Sides of the Placenta
The placenta is not purely a fetal organ or a maternal organ. It’s both. The fetal side develops from the chorion, a membrane that forms from the embryo’s own cells. The maternal side comes from the endometrium itself, which transforms into a specialized tissue called the decidua once pregnancy begins.
The deepest layer of the decidua, right at the implantation site, is called the decidua basalis. This is where the fetal villi anchor into maternal tissue and where the exchange of oxygen, nutrients, and waste products takes place. A thin membrane separates fetal blood vessels from maternal blood, allowing molecules to cross without the blood itself mixing. Think of it as two water systems separated by a very thin, selectively permeable wall.
How the Blood Supply Gets Built
One of the most remarkable things the placenta does is completely remodel the mother’s blood vessels at the implantation site. In the first weeks of pregnancy, trophoblast cells from the embryo invade the walls of small spiral arteries in the uterus. They strip away the smooth muscle and replace the vessel lining with their own cells, essentially hijacking the blood supply.
The result is dramatic. These arteries go from narrow, high-resistance vessels with a diameter of about 200 micrometers to wide, low-pressure channels that can reach 2 millimeters across. That’s a tenfold increase in diameter at the point where they open into the placenta. By the third trimester, this remodeling produces a tenfold increase in blood flow to the placenta compared to a non-pregnant uterus. The mother’s blood then pools in open spaces around the fetal villi, where the exchange happens.
When this remodeling process goes wrong, the consequences can be serious. Incomplete transformation of spiral arteries is linked to conditions like preeclampsia and fetal growth restriction, both of which involve inadequate blood flow to the placenta.
Development Timeline
The placenta doesn’t appear fully formed. It builds gradually over the first trimester and continues maturing throughout pregnancy.
- Weeks 1 to 2: The blastocyst implants in the uterine wall. Trophoblast cells begin invading the endometrium. At this stage, the placenta is a relatively low-oxygen environment.
- Weeks 3 to 8: Villi develop and branch out, establishing the network where blood exchange will occur. Trophoblast cells begin breaching the walls of spiral arteries and remodeling them into larger vessels.
- Weeks 7 to 9: A critical hormonal shift takes place. Up to this point, the corpus luteum (a temporary structure in the ovary) has been producing the progesterone needed to maintain the pregnancy. Around weeks 7 to 9, the placenta takes over progesterone production. This is called the luteal-placental shift, and it’s one reason the risk of miscarriage drops significantly after the first trimester.
- Weeks 10 to 13: Maternal blood flow into the placenta increases as remodeled arteries open up. The placenta becomes the primary organ supporting the pregnancy.
- Second and third trimesters: The villi continue to branch more densely to keep up with the growing fetus’s demand for oxygen and nutrients. The placenta reaches its full size by late pregnancy.
Size and Weight at Full Term
A fully developed placenta at term averages about 500 grams (just over one pound), with a diameter of roughly 18.5 centimeters and a thickness of about 2.3 centimeters. Normal weight ranges from around 300 to 890 grams, depending largely on the baby’s birth weight. The ratio of birth weight to placental weight typically falls around 6:1, meaning the baby weighs about six times more than its placenta.
Placental size matters clinically. An unusually small placenta relative to the baby may indicate the organ struggled to keep up with demand. An unusually large one can signal conditions like gestational diabetes or infection. After delivery, the placenta is often examined for abnormalities that could explain complications that occurred during pregnancy or birth.