The IGFs are believed to be important in pregnancy and are implicated in the pathophysiology of pre-eclampsia. In adults the IGFs circulate primarily with IGF-binding protein-3 (IGFBP-3) and an acid-labile glycoprotein (ALS) in a 140 kDa complex which limits IGF bioavailability. Less than 10% of IGFBP-3 is in lower molecular weight forms. We have investigated the developmental regulation of the IGF/IGFBP system in normal and pre-eclamptic pregnancies with particular emphasis on the IGFBP-3 ternary complex. Circulating levels of IGF-I, IGFBP-3 and ALS, and their degree of association in the ternary complex in the fetus increased with gestational age. In neonatal serum from deliveries <35 weeks' gestation IGFBP-3 was predominantly in 30-50 kDa form(s) and ALS was a limiting factor for ternary complex formation. In serum from deliveries >35 weeks both ALS and IGFs were limiting but approximately 25% of IGFBP-3 was unable to form the ternary complex even in the presence of excess ALS and IGF-I. Serum IGFBP-1, -2 and -6 concentrations tended to decrease with increasing gestational age. In pre-eclamptic pregnancies, amniotic fluid IGFBP-2, -3 and -6 levels decreased with gestational age while IGFBP-1 levels did not show the normal decline. We speculate that the endocrine IGF system develops in the fetus during the third trimester of pregnancy when ALS levels increase.