What is the Amniotic Sac and Why is it Important?
The amniotic sac, also called the bag of waters or the membranes, is the sac in which the embryo and later fetus develops in amniotes. It is a thin but tough transparent pair of membranes that hold a developing embryo (and later fetus) until shortly before birth. The amniotic sac protects and provides nutrients to the fetus, and plays a vital role in fetal development.
Structure and Function of the Amniotic Sac
The amniotic sac consists of two layers: the inner layer, called the amnion, and the outer layer, called the chorion. The amnion encloses the amniotic cavity, which contains the amniotic fluid and the fetus. The chorion is part of the placenta, which connects the fetus to the maternal blood supply.
The amniotic fluid is a clear, slightly yellowish liquid that cushions the fetus from mechanical shocks, regulates temperature, prevents dehydration, and allows fetal movements. The amniotic fluid also contains fetal cells, hormones, proteins, and other substances that can be used for prenatal diagnosis of chromosomal abnormalities and fetal infections .
The amniotic sac develops from the inner cell mass of the blastocyst, which is the early stage of embryonic development. The inner cell mass differentiates into two layers: the epiblast and the hypoblast. The epiblast gives rise to the amnion and the embryo, while the hypoblast gives rise to the yolk sac and later contributes to the formation of the chorion .
Changes in the Amniotic Sac During Pregnancy
The amniotic sac grows and changes along with the fetus throughout pregnancy. At first, it is very small and barely visible. By 12 weeks of gestation, it measures about 12 cm in diameter and contains about 50 ml of fluid. By 20 weeks of gestation, it measures about 20 cm in diameter and contains about 400 ml of fluid. By 37 weeks of gestation, it measures about 25 cm in diameter and contains about 800 ml of fluid.
The amniotic sac usually ruptures during labor, either spontaneously or artificially by a health care provider. This is commonly known as “breaking the water” or “water breaking”. When this happens, some or all of the amniotic fluid leaks out through the vagina. This can be a sign that labor is starting or progressing.
In some rare cases, a baby can be born with part or all of the amniotic sac intact. This is called an “en caul” birth. This can happen when the membranes are very strong or when there is little or no pressure on them during delivery. An en caul birth is not harmful to the baby or the mother, and some cultures consider it a sign of good luck.
Complications Related to the Amniotic Sac
Although the amniotic sac is designed to protect and nourish the fetus, sometimes it can be associated with some complications that can affect fetal health or pregnancy outcome. Some of these complications are:
- Oligohydramnios: This is a condition where there is too little amniotic fluid in the sac. This can cause fetal growth restriction, umbilical cord compression, fetal distress, or premature birth.
- Polyhydramnios: This is a condition where there is too much amniotic fluid in the sac. This can cause maternal discomfort, preterm labor, placental abruption, or fetal malposition.
- Premature rupture of membranes (PROM): This is a condition where the amniotic sac breaks before labor starts. This can increase the risk of infection for both mother and baby, as well as preterm birth or umbilical cord prolapse.
- Chorioamnionitis: This is a bacterial infection of the amniotic sac and/or fluid. This can cause fever, uterine tenderness, foul-smelling vaginal discharge, or fetal tachycardia. It can also lead to serious complications such