Objective: The purpose of this study was to determine the concentrations of serum insulin-like growth factor-I and insulin-like growth factor binding protein-1 in pregnancies that are complicated with primary antiphospholipid syndrome. Study Design: Longitudinal blood samples were collected from 8 weeks of gestation in 28 women with treated primary antiphospholipid syndrome and 19 control women. Serum insulin-like growth factor-I and insulin-like growth factor binding protein-1 were measured by immunoradiometric assay. Results: Three antiphospholipid syndrome pregnancies miscarried, four pregnancies had intrauterine growth restriction and preeclampsia, six pregnancies had a thrombotic event, and one pregnancy had abruptio placentae. Mean (±SD) birth weight in antiphospholipid syndrome group was 2867 ± 914 g (control, 3492 ± 527 g; P = .02), and the mean gestation at delivery was 36.5 ± 5.2 weeks (control, 40.3 ± 0.7 weeks; P = .002). Insulin-like growth factor binding protein-1 and insulin-like growth factor-I concentrations increased with gestational age in both groups (2.8% and 2.4% per week), but insulin-like growth factor binding protein-1 was 61% higher in the antiphospholipid syndrome group (95% CI, 16%-122%; P = .004). Insulin-like growth factor-I was not significantly different (8% higher in antiphospholipid syndrome; 95% CI, −10% to 30%; P = .41). Conclusion: Serum concentrations of insulin-like growth factor binding protein-1 are abnormal in the antiphospholipid syndrome group and may reflect abnormalities in trophoblast invasion. (Am J Obstet Gynecol 2003;189:274-9.)