Amniotic Sac

The amniotic sac, often referred to as the bag of waters or simply the membranes, is a transparent yet resilient pair of membranes that surrounds and protects the developing embryo and fetus of amniotes. Formed early in gestation, it provides a fluid-filled environment in which the fetus can grow, move and develop safely. Together with the yolk sac, allantois and chorion, the amniotic sac forms part of the extraembryonic structures essential for nourishment, protection and metabolic exchange throughout prenatal development.

Structure

The amniotic sac is composed of two closely apposed layers:

• The amnion, the inner membrane, which lines the amniotic cavity and directly encloses the amniotic fluid and fetus.
• The chorion, the outer membrane, which surrounds the amnion and contributes to the formation of the placenta.

Externally, the amniotic sac is linked to the yolk sac, allantois and the placenta via the umbilical cord. These connections ensure metabolic exchange, waste removal and mechanical protection.
The amniotic cavity is the closed space between the embryo and the amnion, containing the amniotic fluid. It forms as the amniotic folds—initially emerging at the cephalic end and later at the caudal and lateral margins—rise and fuse above the embryo, enclosing it dorsally.

Development

During implantation at the beginning of the second week, a small cavity appears within the inner cell mass. As this enlarges, it becomes the early amniotic cavity. The epiblast forms the floor of this cavity, while flattened amnioblasts, derived from the epiblast, line the inner surface of the amnion. The hypoblast forms the roof of the primary yolk sac. As development progresses, extraembryonic mesoderm surrounds both the amnion and yolk sac.
The amnion develops soon after the yolk sac and remains a key extraembryonic membrane throughout gestation. By about a month, the chorion surrounds the embryo and later fuses with the amnion as the amniotic cavity enlarges. The amnion expands rapidly as amniotic fluid accumulates, filling the sac and pressing against the chorionic layer.

Birth

Occasionally, infants are born with their heads or bodies still encased in the intact amniotic sac—a condition known as being born in a caul. Under light the sac appears shiny, smooth and tough. After birth, the sac, along with the placenta and umbilical cord, is expelled as part of the afterbirth.

Function

The amniotic sac performs several essential functions throughout gestation:

• Protection: The amniotic fluid cushions the fetus, reducing the risk of injury from external pressure or trauma.
• Mobility: The buoyant environment permits free fetal movement, promoting musculoskeletal development.
• Exchange: The sac assists in the exchange of oxygen and other essential substances between the fetus and the placenta via the umbilical cord.
• Temperature regulation: The fluid environment helps maintain a stable thermal environment for the fetus.

By providing both mechanical and biochemical support, the amniotic sac is central to normal fetal development.

Clinical Significance

Inflammation of the membranes, known as chorioamnionitis, typically caused by infection, is a significant risk factor for neonatal sepsis. Diagnosis and management require prompt clinical attention.
The rupture of membranes (ROM) is a key event in labour. Normally it occurs spontaneously at term, either just before or during labour. Premature rupture of membranes (PROM) refers to rupture occurring before the onset of labour. When rupture is performed deliberately—usually to induce or expedite labour—the procedure is termed artificial rupture of membranes (AROM) or amniotomy, typically carried out with an amnihook or amnicot.
The amniotic sac must also be punctured during amniocentesis, a routine diagnostic procedure performed in the second trimester. Although generally safe, this can introduce a small risk of infection. In most cases, ascending vaginal infection is a more common cause of amniotic infection than procedures such as amniocentesis.