The placenta is a temporary organ that develops during pregnancy, serving as the interface between the mother and the developing fetus. It forms shortly after the blastocyst implants into the uterine wall. This specialized structure provides the fetus with sustenance and maintains its internal environment throughout gestation. The placenta is expelled from the body after delivery.
How the Placenta Develops and Is Structured
The placenta forms immediately following implantation, involving contributions from both maternal and fetal tissues. The maternal portion arises from the decidua basalis, the modified uterine lining at the implantation site. The fetal component develops from the chorion frondosum, the outer layer of the blastocyst.
The developed organ has two surfaces. The fetal side, or chorionic plate, is smooth, covered by the amnion, and contains the umbilical cord insertion. The maternal side, or basal plate, is rough and segmented into 15 to 20 bulging areas called cotyledons.
The functional unit is the chorionic villi, finger-like projections extending into the maternal blood supply. These villi are bathed in maternal blood within the intervillous space. A thin layer of tissue, called the placental barrier, separates the maternal blood from the fetal capillaries. This barrier ensures the two circulatory systems remain physically separate.
The Placenta’s Functions During Pregnancy
The placenta acts as surrogate organs for the fetus, performing the roles of the lungs, kidneys, and digestive system. Its primary role is the transfer and exchange of substances between the two separate bloodstreams. Oxygen transfers from the mother to the fetus, while carbon dioxide moves in the opposite direction, functioning as the fetal lungs.
Nutrient and Waste Transfer
Nutrient delivery supports rapid fetal growth. Simple sugars, like glucose, which is the fetus’s main energy source, pass across the barrier via facilitated transport mechanisms. Amino acids, needed for protein synthesis, are actively transported, often resulting in higher concentrations in fetal circulation than in the mother’s. Metabolic waste products, including urea and creatinine, are transferred into the maternal circulation for excretion.
Immunological Protection
The placenta manages a complex immunological role, preventing the mother’s immune system from attacking the fetus. It selectively transfers maternal antibodies, specifically immunoglobulin G (IgG), to the fetus. This transfer provides passive immunity, offering protection against certain infections during the first few months after birth.
Endocrine Function
The organ serves as an endocrine gland, producing hormones that maintain the pregnancy and prepare the mother’s body for birth and lactation. Human Chorionic Gonadotropin (hCG) signals pregnancy and ensures continued progesterone production. Progesterone and estrogen are produced in large amounts, supporting the uterine lining and promoting growth. Human Placental Lactogen (hPL) aids in fetal metabolism and prepares the mammary glands for milk production.
Potential Pregnancy Complications Involving the Placenta
Abnormalities in placental placement or attachment can lead to complications for both the mother and the fetus.
Placenta Previa and Abruption
Placenta previa occurs when the placenta implants low in the uterus, partially or completely covering the cervix opening. This condition causes painless bleeding in the second or third trimester and often requires a cesarean delivery to avoid hemorrhage. Placental abruption is the premature separation of the placenta from the uterine wall before birth. Abruption results in bleeding and a loss of oxygen and nutrients to the fetus, potentially leading to preterm birth or fetal growth restriction.
Placenta Accreta Spectrum (PAS)
PAS involves an abnormal depth of placental attachment to the uterine wall. Normally, the placenta detaches easily after birth, but in PAS, the tissue invades the muscular layer of the uterus. PAS disorders carry a high risk of massive hemorrhage during delivery, often requiring a hysterectomy to control blood loss. Placental health is also closely associated with disorders like preeclampsia and fetal growth restriction, where dysfunction in the organ can contribute to these conditions.
The severity of PAS is categorized by the depth of invasion:
- Placenta accreta: The placenta attaches directly to the uterine muscle.
- Placenta increta: The tissue invades deeper into the muscle itself.
- Placenta percreta: The tissue penetrates through the entire uterine wall and may invade nearby organs, such as the bladder.
Delivery and Postpartum Handling
The delivery of the placenta occurs after the baby has been born. The uterus contracts again, causing the organ to shear away from the uterine wall. Signs of detachment include a small gush of blood and the lengthening of the umbilical cord.
The placenta is expelled, typically within 30 minutes of delivery. This process can be managed physiologically, using the mother’s own contractions, or actively, using medication to speed up expulsion and reduce bleeding risk. Healthcare providers inspect the delivered placenta to ensure it is complete and that no fragments remain in the uterus, which could cause infection or hemorrhage.
Following inspection, the placenta is usually disposed of as clinical waste according to hospital protocols. Some families choose to keep the organ for personal reasons. Alternative uses may involve burial, artistic preservation, or placentophagy, the practice of ingesting the placenta.