Human placental lactogen (hPL; chorionic somatomammotropin) is a 21,000 KD polypeptide produced during pregnancy by placental trophoblastic cells. The level of hPL in maternal serum is directly related to placental function and fetal well-being. hPL is detected at about 6 weeks after conception and its concentration increases gradually to peak levels (without decreases) until about the 34th week where it remains stable for the remainder of the pregnancy. Consistently low levels throughout pregnancy or a sudden drop in serial determinations are an indication of fetal distress. After normal delivery, the hPL concentration falls rapidly to an undetectable level. The hPL levels in serum of women with multiple placenta pregnancies generally exceeds that of single placenta pregnancies. This is generally noted from the 2nd trimester to delivery.