Introduction

Amniotic fluid surrounds the embryo and fetus during development and has a myriad of functions. Physically, it protects the fetus in the event the maternal abdomen is the object of trauma. Furthermore, it protects the umbilical cord by providing a cushion between the fetus and the umbilical cord, thus reducing the risk of compression between the fetus and the uterine wall.[1] Amniotic fluid also helps protect the fetus from infectious agents due to its inherent antibacterial properties. Additionally, it serves as a reservoir of fluid and nutrients for the fetus containing: proteins, electrolytes, immunoglobulins, and vitamins from the mother. It provides the necessary fluid, space, and growth factors to allow normal development and growth of fetal organs such as the musculoskeletal system, gastrointestinal system, and pulmonary system.[2]  Clinicians can use amniotic fluid as a tool to monitor the progression of pregnancy and predict fetal outcomes.

Development

The development of amniotic fluid organizes into early gestation and late gestation. Early gestation is the embryonic period which is from the start of fertilization to 8 weeks, and late gestation, which encompasses the fetal period 8 weeks to birth.  The composition of amniotic fluid changes from early gestation to late gestation. During the embryonic period, amniotic fluid derives from both fetal and maternal factors such as water from maternal serum, coelomic fluid, and fluid from the amniotic cavity; however, during late gestation, amniotic fluid is largely produced by fetal urine and lung secretions.[3][4]

Early Gestation

In early gestation, two fluid-filled sacs surround the embryo: the exocoelomic cavity and the amniotic cavity. The formation of the coelomic cavity begins during the fourth week of gestation when the exocoelomic cavity splits the extraembryonic mesoderm into the splanchnic mesoderm lining and the somatic mesoderm. The coelomic fluid within the coelomic cavity stays in direct contact with the mesenchyme of the developing placenta villi during the first trimester. Before it disappears, the coelomic cavity acts as a transfer area as well as a reservoir of nutrients for the growing embryo. The exocoelomic cavity forms inside the extraembryonic mesoderm alongside the placental chorionic plate and is now believed to be an essential transfer interface and reservoir of nutrients for the embryo because coelomic fluid has shown to have ultrafiltrate of maternal serum as well as products derived from the placenta and secondary yolk sac. This arrangement suggests that the coelomic fluid is essentially an extension of the placenta, providing the embryo with nutrients until the amniotic cavity becomes large enough to take over later in development.[2][5] Gradually, the coelomic cavity shrinks as the amniotic cavity expands and completely disappears by week 12.[5][6] At this point in development, the primary function of amniotic fluid is the expansion of the amniotic sac, which allows room for the fetus to grow unimpeded.[1]

Late Gestation

Once the coelomic fluid begins to disappear, the amniotic cavity takes over. In the early stages of gestation, the water in amniotic fluid is derived mostly from maternal serum; however, at 10 weeks, the fetus begins to produce urine which is secreted into the amniotic sac. During late gestation (the second and third trimesters), as the amniotic fluid expands, fetal urine becomes the largest source of the amniotic fluid.[6] Lung secretions, gastrointestinal secretions, and excretions from the umbilical cord and placental surface contribute to the composition of amniotic fluid as well; however, lung secretions alone make up as much as one-third of amniotic fluid.[4][7] Compared to the composition of yellow coelomic fluid early in pregnancy, amniotic fluid is less viscous and always clear due to its lower protein concentration. Amniotic fluid is 98% water and electrolytes, and signaling molecules, peptides, carbohydrates, lipids, and hormones make up the other 2%.[2][4]

Pathophysiology

Homeostasis of body fluids is important in the growing fetus. In addition to the constant circulation of amniotic fluid through inhalation and exhalation, there must be a balance between fluid formation and elimination.[4] The formation originates from fetal urine and lung secretions; however, elimination which is important for balance and homeostasis, is largely the result of fetal swallowing and intramembranous absorption.[7][8] Early in pregnancy, embryonic skin is just simple epithelium, allowing fluid to pass freely under hydrostatic and osmotic forces.[3] Furthermore, its composition is similar to the fetus and maternal serum; it freely diffuses through fetal skin and the chorionic villi until week 8. Eventually, fetal skin begins to become stratified epithelium and becomes fully keratinized by 25 weeks.[4][9] Once the skin of the fetus is fully keratinized later in pregnancy, it can no longer absorb or transfer fluids as easily back and forth. Respiration, swallowing, and urination are the main routes of exchange between the fetus and amniotic fluid to maintain the fluid balance.[3] The two largest contributors to elimination are fetal swallowing and the intramembranous pathway. Though there are many mechanisms for eliminating amniotic fluid, the greatest contributor to amniotic fluid elimination is through fetal swallowing, seen as early as 11 weeks.[10][11]

Clinical Significance

Abnormally high or low amniotic fluid volumes have been shown to predict poor fetal outcomes; therefore, a normal amount of amniotic fluid volume is crucial to the healthy development of the fetus or embryo. Amniotic fluid has proven to be a major diagnostic tool when monitoring the progression and health of a pregnancy. Clinicians can use the amniotic fluid index (AFI) or single deepest pocket (SDP).[12][13] These measurements are part of the biophysical profile that consists of fetal tone, fetal breathing, and a non-stress test.  AFI and SDP are estimations of amniotic fluid volume based on ultrasound measurements. An AFI of greater than 24 cm or an SDP of more than 8 cm is considered polyhydramnios which is an increased amount of amniotic fluid. Polyhydramnios can be caused by gastrointestinal tract obstruction, genetic disorders, musculoskeletal disorders, or congenital diaphragmatic hernias.[13] Conversely, oligohydramnios is an AFI under 5 cm or SDP less than 2 cm.[14] Oligohydramnios can cause complications such as renal agenesis, genitourinary tract obstruction, and IUGR.[15][16] Clinicians can also use hormones, peptides, and amniotic fluid proteins to screen for genetic diseases.[2] Additionally, invasive testing may be required to obtain information rather than ultrasound, coined an amniocentesis. An amniocentesis is a procedure performed after 15 weeks that takes a sample of amniocytes and is used to diagnose chromosomal abnormalities such as Trisomy 21 (Down syndrome). This procedure, however, is much more invasive than other screening tests and can result in spontaneous miscarriage in 0.5 to 1% of pregnancies.[17] Further studies on the amniotic fluid are in motion; it remains a vital substance required for the embryo or fetus to survive and helps clinicians make decisions regarding are and predict outcomes of pregnancies.