The placenta is a temporary organ that forms during pregnancy, serving as an interface between the pregnant individual and the developing fetus. Understanding its formation timeline and various functions offers insight into the complex processes sustaining pregnancy.
Understanding the Placenta
The placenta develops within the uterus during pregnancy. It has a disc-like shape and connects to the fetus through the umbilical cord. This organ acts as the primary interface, facilitating exchange between the maternal and fetal bloodstreams without them directly mixing.
The placenta arises from both fetal and maternal components, with the fetal portion forming from the chorion frondosum and the maternal portion from the decidua basalis. It serves as a life-support system, ensuring necessary resources for growth and development.
The Placenta’s Developmental Timeline
The development of the placenta begins after the fertilized egg implants in the uterine wall, around 7 to 10 days after conception. At this stage, specialized cells from the blastocyst, called trophoblasts, burrow into the uterine lining, forming the initial building blocks of the placenta.
Trophoblasts differentiate into two layers: the inner cytotrophoblast and the outer syncytiotrophoblast. The syncytiotrophoblast forms small spaces called lacunae, which quickly fill with maternal blood, establishing early uteroplacental circulation by the end of the second week of gestation. Cytotrophoblast cells then form finger-like projections, primary chorionic villi, extending into the syncytiotrophoblast.
By the third week, extra-embryonic mesoderm grows into these primary villi, transforming them into secondary chorionic villi. Development progresses by the fourth week, as embryonic blood vessels form within the mesoderm of these villi, creating tertiary villi. These vascularized villi lay the foundation for the exchange of substances between the mother and the embryo.
The placenta is considered structurally formed and functionally mature, with its primary structure and blood flow established, between 10 and 12 weeks of gestation. At this point, it takes over hormone production from the corpus luteum. Although functionally mature, the placenta continues to grow in size and weight throughout pregnancy to accommodate the increasing demands of the developing fetus. By 10 to 12 weeks, the average placenta weighs nearly 2 ounces, and by term, it will weigh approximately 1 pound. The full development of the maternal blood supply to the placenta is completed by the end of the first trimester.
Essential Placental Functions
Once formed, the placenta performs functions important for fetal growth and pregnancy. It acts as a selective barrier and transport system, facilitating the exchange of substances between the mother and the fetus without directly mixing their blood.
A primary function is the transfer of oxygen and nutrients, such as glucose, amino acids, fatty acids, vitamins, and minerals, from the maternal bloodstream to the fetus. Simultaneously, the placenta removes waste products, including carbon dioxide and urea, from fetal blood and transfers them to the mother’s circulatory system for excretion. This dual action ensures the fetus receives nourishment and eliminates metabolic byproducts.
The placenta is also an endocrine organ, producing hormones that regulate pregnancy and support fetal development. Human chorionic gonadotropin (hCG) is one such hormone, produced early in pregnancy to maintain the corpus luteum, which ensures continued progesterone production. Progesterone helps sustain the uterine lining and prevents premature uterine contractions, while estrogen stimulates uterine growth, aids mammary gland development, and influences placental vascular growth.
Beyond nutrient and hormone exchange, the placenta provides immune protection to the fetus. It transfers maternal antibodies, specifically immunoglobulin G (IgG), which confer passive immunity against various infections. The placenta also acts as a physical and immunological barrier, shielding the fetus from certain pathogens and modulating the maternal immune response to prevent rejection of the genetically distinct fetus.