Amnionic luid volume increases from approximately 30 mL at 10 weeks to 200 mL by 16 weeks and reaches 800 mL by the mid-third trimester (Brace, 1989; Magann, 1997). The luid is approximately 98-percent water. A full-term fetus contains roughly 2800 mL of water and the placenta another 400 mL, such that the term uterus holds nearly 4 liters of water (Modena, 2004). Abnormally decreased luid volume is termed oligohydramnios, whereas abnormally increased luid volume is termed hydramnios or poyhydramnios.
Early in pregnancy, the amnionic cavity is filled with luid that is similar in composition to extracellular luid. During the first half of pregnancy, transfer of water and other small molecules takes place across the amnion-transmembranous low; across the fetal vessels on placental surface-intramembranous low; and transcutaneous low-across fetal skin. Fetal urine production begins between 8 and 11 weeks' gestation, but it does not become a major component of amnionic luid until the second trimester, which explains why fetuses with lethal renal abnormalities may not manifest severe oligohydramnios until after 18 weeks. Water transport across the fetal skin continues until keratinization occurs at 22 to 25 weeks. This explains why extremely preterm neonates can experience signiicant luid loss across their skin.
With advancing gestation, four pathways playia major role in amnionic luid volume regulation (Table
First, fetal urination is the primary source of amnionic luid in the second half of pregnancy. By term, fetal urine production may exceed 1 liter per day, and the entire amnionic luid volume is recirculated on a daily basis. Fetal urine osmolality is similar to that of amnionic luid and significantly hypotonic to that of maternal and fetal plasma. Speciically, the osmolality of maternal
TABLE 11-1. Amnionic Fluid Volume Regulation in Late Pregnancy
Data from Magann, 201l1; Modena, 2004; Moore, 201l0.
and fetal plasma approximates 280 mOsm/mL, whereas that of amnionic fluid is about 260 mOsm/L. The hypotonicity of amnionic luid accounts for significant intramembranous fluid transfer across and into fetal vessels on the placental surface. This transfer reaches 400 mL per day and is a second regulator of fluid volume (Mann, 1996). In the setting of maternal dehydration, the resultant increase in maternal osmolality favors fluid transfer from the fetus to the mother, and then from the amnionic luid compartment into the fetus (Moore, 2010).
n important third source of amnionic fluid regulation is the respiratory tract. Approximately 350 mL of lung fluid is produced daily late in gestation, and half of this is immediately swallowed. Last, fetal swallowing is the primary mechanism for amnionic luid resorption and averages 500 to 1000 mL per day (Mann, 1996). Impaired swallowing, secondary to either a central nervous system abnormality or GI tract obstruction, can result in an impressive degree of hydramnios. The remaining pathways are transmembranous and transcutaneous low, which together account for a far smaller proportion of fluid transport in the second half of pregnancy.
From a practical standpoint, the actual volume of amnionic luid is rarely measured outside of the research setting. That said, direct measurement and dye-dilution methods of luid quantification have contributed to our understanding of normal physiology. These measurements have further been used to validate sonographic fluid assessment techniques. Dye dilution involves injecting a small quantity of a dye such as aminohippurate into the amnionic cavity under sonographic guidance and then sampling the amnionic fluid to determine the dye concentration and hence to calculate the volume.