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Abstract
Alpha fetoprotein (AFP) measurement is possible by rocket immunoelectrophoresis and counterimmunoelectrophoresis methods which can measure concentrations as low as .1-1 mg/ml. A several 100-fold difference in AFP between fetal serum and amniotic fluid (AF) exists; hence cross-contamination can cause false positive results. 90% of proteins in AF however come from the maternal serum (MS) compartment and since AFP is small enough to cross into the MS compartment its levels can be used to detect prenatally fetal defects by measuring MS AFP levels. In second trimester AFP falls rapidly because normal AF volume is expanding and fetal production rates remain relatively constant. Polyhydramnios may change AFP in either direction (e.g. depressed with maternal diabetes and elevated with fetal swallowing defect). AF AFP is elevated in some cases of twin pregnancy; similarly MS AFP is more constantly elevated than AF. For proper laboratory results accurate pregnancy date estimation is essential because of rapidly changing second trimester AFP values. If careful attention is given to possible fetal blood contamination pregnancy date and twin pregnancy AF AFP measures are 90% accurate or better in mothers at high risk for neural tube defects (NTD). MS AFP is measured in units 1000-fold lower than AF (mcg/ml) and 1 million-fold lower than fetal serum (mg/ml). MS AFP does not rise above nonpregnancy until Week 10 of gestation. Elevated MS AFP in first trimester is probably due to direct fetomaternal transfer (placental injury or separation). AFP in MS reaches maximum 30 weeks postconception or 30-32 weeks of pregnancy. 3 problems in comparing and applying AFP screening results are: 1) differing definitions of upper limits of normal pregnancy AFP values; 2) differing incidence of NTD in populations studied; and 3) possibility of genetic variations in normal pregnancy values. Low for pregnancy date values indicate decidua without remaining fetal parts.
Published Version
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