Fetal membranes are specialized tissues that surround and protect a developing fetus throughout pregnancy. Their presence is fundamental for a healthy pregnancy, providing an environment that supports growth and safeguards the fetus from external forces and potential harm.
What Fetal Membranes Are
Fetal membranes are extraembryonic membranes that enclose the embryo and fetus. They develop from the zygote and are not considered maternal tissue. These membranes generally position themselves within the uterus, forming a protective sac around the developing life. There are four main types of fetal membranes: the amnion, chorion, yolk sac, and allantois.
Roles of Individual Membranes
The amnion is the innermost membrane directly enclosing the fetus and the amniotic fluid. This thin, transparent membrane produces the amniotic fluid, which fills the amniotic sac.
The chorion is the outer membrane, positioned external to the amnion. The chorion plays a significant role in forming the fetal part of the placenta. It features chorionic villi, which are extensions that invade the uterine lining, establishing a connection for nutrient and waste exchange between mother and fetus.
The yolk sac is a small, membranous structure outside the embryo, connecting to the midgut via the yolk stalk. In humans, it does not contain yolk. It provides temporary nourishment, facilitates gas exchange, and is a primary site for early blood cell formation. The yolk sac also produces germ cells and contributes to the formation of the gastrointestinal system and reproductive organs. This structure typically regresses by about 15 weeks of gestation.
The allantois is an outgrowth from the caudal end of the yolk sac. While small and somewhat regressed in humans, it contributes to the development of the umbilical cord by forming its blood vessels, the umbilical arteries and vein. The base of the allantois also contributes to the formation of the upper part of the urinary bladder.
How Membranes Protect and Support Fetal Development
The amniotic fluid, contained within the amnion, acts as a shock absorber, protecting the fetus from external trauma and mechanical pressures. This fluid also helps maintain a consistent temperature around the fetus, creating a stable thermal environment for its development. The fluid volume, typically ranging from 500 to 1000 ml at term, allows for free fetal movement, which is important for proper musculoskeletal development. Fetal breathing movements, involving the inhalation and swallowing of amniotic fluid, are also crucial for lung and digestive system development.
The chorion and amnion work together to form a barrier against ascending infections from the vagina. The chorion, with its villi, forms the fetal part of the placenta, facilitating the transfer of oxygen and nutrients from the mother’s blood to the fetus. Simultaneously, waste products from the fetus are transferred back to the mother for excretion. The rupture of these membranes, commonly known as “water breaking,” is a natural event signaling the onset of labor. This event increases prostaglandin production and decreases cushioning, both of which contribute to the frequency and intensity of uterine contractions.
Common Conditions Affecting Fetal Membranes
Premature Rupture of Membranes (PROM) occurs when the amniotic sac breaks before labor begins. If this happens before 37 weeks of pregnancy, it is termed preterm PROM (PPROM) and is a significant cause of preterm births. PROM increases the risk of infection for both the mother and the fetus, with the risk rising the longer the membranes remain open. Other implications include umbilical cord compression and placental abruption.
Chorioamnionitis is an infection and inflammation of the fetal membranes and amniotic fluid. This condition is typically caused by bacteria, such as E. coli and group B streptococci, that ascend from the vagina or anus. Risk factors include prolonged labor, premature rupture of membranes, and frequent vaginal examinations after the water breaks. Chorioamnionitis can lead to complications such as preterm birth, maternal sepsis, and neonatal infections like pneumonia or meningitis.
Amniotic fluid disorders involve an abnormal volume of amniotic fluid. Polyhydramnios refers to an excessive accumulation of amniotic fluid, occurring in about 1% of pregnancies. This can lead to preterm labor, premature rupture of membranes, and may indicate fetal anomalies. Conversely, oligohydramnios is characterized by insufficient amniotic fluid, affecting approximately 11% of pregnancies. Too little fluid can result in poor fetal lung development, umbilical cord compression, and delayed fetal growth.