In an unselected obstetric population of 869 women serial determinations of estriol in serum and urine were performed from the 28th week of pregnancy until delivery. Clinical management was based on ultrasound and nonstress/contraction stress tests only. Data on the development of the infants were available after 1 year in 759 cases (89%) and after 2 years in 661 cases (78%). Serum free estriol (E3) screening during weeks 28-34 of pregnancy revealed a significantly increased risk for reduced Apgar scores, growth retardation and postnatal complications in pregnancies with decreased levels (p less than 0.001). The development of the children was disturbed by a higher incidence of childhood diseases, retardation in speech and bowel and bladder control. The urinary estrogen determinations (UE) during this period of pregnancy showed only a vague connection with birth weight and Apgar scores (p less than 0.05) and no connection to the infant's development. Serial determinations of E3 after the 35th week of pregnancy increased the significance for all parameters tested. If the estrogen concentration was determined in the last 2 weeks before delivery, 50% of the SGA and 60% of the endangered cases could be diagnosed. After reduced E3 serial levels neurological sequelae, reduced body weight, retarded speech and late development of bowel and bladder control were significantly more frequent at age two than after normal E3 levels. The differences obtained by serial UE determinations were less evident. Considering cost-benefit-calculations, an E3 screening of every pregnant woman cannot recommended. In pregnancies at risk serial E3 determinations allow better prognostication of fetal well-being. In the case of reduced E3 values maximum post partum care should be made available for all newborns. Special support should be given to the early infant's development after reduced E3 values have been observed.