Amniotic fluid (AF) is an active component of the intrauterine environment and serves multiple roles. AF plays a vital part in normal fetal development. Its functions include:
maintaining a constant intrauterine temperature,
cushioning the fetus against trauma,
providing space for fetal movement and growth, and
serving as a reservoir for fetal waste products before maternal excretion.
AF has considerable diagnostic utility. Abnormalities in AF volume can indicate various fetal or maternal pathologies, and as a convenient source of fetal cells, AF is commonly collected for prenatal genetic evaluation. Finally, AF plays a not yet fully understood role in one of the most devastating perinatal complications, the AF embolism (AFE) syndrome.
The AF system is a dynamic one, with current research indicating that the entire volume of fluid turns at least once a day. The rate of change and total volume of AF correlates with gestational age. AF volume increases considerably between 8 and 30 weeks gestation, peaks in weeks 32–34, and declines thereafter.
At any given time, AF volume is reflective of the balance between fluid production and resorption mechanisms, though the regulation of these mechanisms is not understood. In pathologic states, wherein these mechanisms are altered, AF volume can change quickly and considerably. Excessive amounts of AF (polyhydramnios) may total several liters, or in conditions of low fluid (oligohydramnios), AF volumes may approach zero. As will be further discussed below, abnormal fluid volumes may result from various fetal anomalies and maternal disease states.
Amniotic Fluid Production
Early AF is thought to arise as an ultrafiltrate of maternal plasma. At the start of the second trimester, the major source of AF is the fetal plasma, which diffuses through the fetal skin until keratinization after about 20 weeks gestation. Around 9–12 weeks gestation, the fetal kidney begins to excrete urine, and by the time of keratinization, fetal urine constitutes the majority of AF. Fluid produced by the fetal lungs is a minor contributor to AF volume, and fluid transudation across the placenta and umbilical cord accounts for the smallest portion.
Amniotic Fluid Resorption
The two major mechanisms of AF resorption are thought to be fetal swallowing and intramembranous absorption. Swallowing, which begins early in gestation, is believed to account for the bulk of AF removal. However, fetal urine and lung fluid production occur at greater volumes than swallowing has been shown to remove. The proposed mechanism for resorption of the remainder of fluid is via intramembranous routes. AF becomes increasingly hypotonic with advancing gestation, so the growing osmotic gradient between AF and fetal blood is thought to pull AF into fetal circulation at the fetal surface of the placenta. While this process is still being studied, it is thought that intramembranous absorption may be the primary regulator of AF volume; ...