Placenta formation, or placentation, begins soon after fertilization and implantation. Following the fusion of the oocyte and sperm in the fallopian tube, the zygote undergoes rapid mitotic divisions called cleavage as it travels toward the uterine cavity. By day three, it forms a solid ball of cells known as a morula, which subsequently develops into a fluid-filled blastocyst by day five. The blastocyst consists of an inner cell mass that will form the embryo and an outer layer of cells called the trophoblast, which drives the entire placental development process.
Implantation occurs around the sixth to tenth day after fertilization. The blastocyst burrows into the functional layer of the endometrium, now termed the decidua. Under the influence of progesterone, endometrial stromal cells undergo decidualization, enlarging and accumulating glycogen and lipids to nourish the early conceptus.
As the blastocyst embeds into the endometrium, the trophoblast differentiates into two distinct layers:
Projecting outward from these layers, primary chorionic villi appear as cellular extensions. These villi evolve into secondary and tertiary structures containing fetal capillary loops, establishing the crucial structural framework for nutrient and gas exchange between mother and baby.
By the end of the first trimester, maternal blood vessels are extensively remodeled by invading trophoblasts. Spiral arteries in the myometrium lose their muscular walls, transforming from high-resistance, low-capacity vessels into low-resistance, high-capacity channels. This physiological adaptation allows maternal blood to flow freely into the intervillous spaces, bathing the fetal chorionic villi. Here, maternal and fetal blood remain separated by a thin placental membrane, allowing efficient diffusion of oxygen and nutrients without direct blood mixing.
At term, the human placenta is a discoid organ weighing about 500 grams. It features two distinct surfaces: the fetal surface, which is smooth and covered by the amnion with the umbilical cord inserting near the center, and the maternal surface, which is rough, dark red, and divided into 15 to 20 bulging areas called cotyledons. Each cotyledon contains several main stem villi and their branches, supplied by branches of the uterine vessels. The structural integrity of these cotyledons is critical; retained fragments following delivery can cause severe postpartum hemorrhage.
The mature placenta acts as the lungs, gut, and kidneys for the developing fetus.
| Function Category | Mechanism / Description |
|---|---|
| Respiratory | Oxygen and carbon dioxide cross the placental membrane freely via simple diffusion. |
| Nutritive | Glucose, amino acids, vitamins, and electrolytes are transported actively or passively to the fetus. |
| Excretory | Fetal metabolic waste products like urea, uric acid, and creatinine cross into maternal blood for excretion. |
| Endocrine | Secretes crucial hormones including hCG, human placental lactogen, estrogen, and progesterone to maintain pregnancy. |
Abnormalities in placental attachment or structure can lead to serious obstetric complications. Placenta previa occurs when the blastocyst implants in the lower uterine segment, partially or completely covering the internal cervical os and causing painless vaginal bleeding in the third trimester. Placenta accreta involves abnormal trophoblastic invasion past the decidua into the myometrium without an intervening decidual layer, which can be further categorized as increta or percreta depending on depth of invasion. These conditions frequently result in life-threatening hemorrhage at delivery.