['To assess the impact of elevated peak serum E(2) levels (EPE(2); defined as levels >90th percentile) on the day of hCG administration during controlled ovarian hyperstimulation (COH) for IVF on the likelihood for small for gestational age (SGA), preeclampsia (PreE), and preterm delivery (PTD) in singleton pregnancies.', 'Retrospective cohort study.', 'Tertiary-care academic medical center.', 'Singleton live-birth pregnancies conceived after fresh IVF-ET.', 'None.', 'The delivery rate of SGA infants and the development of PreE and PTD in patients with and without EPE(2).', 'Patients with EPE(2) during COH were more likely to deliver SGA infants (7 [26.9%] vs. 10 [3.8%]; odds ratio [OR], 95% confidence interval [CI] {9.40, 3.22-27.46}) and develop PreE (5 [18.5%] vs. 12 [4.5%]; adjusted OR, 95% CI {4.79, 1.55-14.84}). No association was found between EPE(2) and the likelihood for delivery before 37 weeks, 35 weeks, or 32 weeks of gestation. Receiver operating characteristic analysis revealed that EPE(2) level predicted adverse obstetrical outcome (SGA + PreE) with 38.5% and 91.7% sensitivity and specificity, respectively. Using a serum peak E(2) cutoff value of 3,450 pg/mL (>90th percentile level), the positive predictive value was 37%, while the negative predictive value was 92%.'] Is peak serum estradiol level during controlled ovarian hyperstimulation associated with increased risk of small for gestational age and preeclampsia in singleton pregnancies after in vitro fertilization?