A search for biochemical markers of fetal status led to development of maternal serum screening tests. One of the first of these tests assessed serum alpha-fetoprotein (AFP) concentrations. AFP is produced normally by the fetal liver, peeks at approximately 14 weeks, and "leaks" into the maternal circulation via the placenta. Thus, AFP concentrations increase in maternal serum during the second trimester and then begin a steady decline after 30 weeks of gestation. In cases of neural tube defects and several other abnormalities, including omphalocele, gastroschisis, bladder exstrophy, amniotic band syndrome, sacrococcygeal teratoma, and intestinal atresia, AFP levels increase in amniotic fluid and maternal serum. In other instances, AFP concentrations decrease as, for example, in Down syndrome, trisomy 18, sex chromosome abnormalities, and triploidy. These conditions are also associated with lower serum concentrations of human chorionic gonadotropin (hCG) and unconjugated estriol. Therefore, maternal serum screening provides a relatively noninvasive technique for an initial assessment of fetal well being.
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