['To determine whether nulliparous women > 40 years old with singleton pregnancies who conceived after infertility treatment are at an increased risk for cesarean section compared with older nulliparous patients who conceived spontaneously.', 'All subjects in this study were nulliparous women > 40 years old with singleton gestations who were delivered of their infants between 1990 and 1998. The Mantel-Haenszel procedure was used to obtain the weighted odds ratios and to control for confounding variables.', 'During the study period, 115 nulliparous women > 40 years old with singleton pregnancies were delivered of their infants in our institute. Of those, 80 pregnancies were spontaneous and 35 pregnancies occurred after infertility treatment. Women treated for infertility had a higher rate of low-birth-weight (< 2500 g) newborns (34.3% versus 10.1%; odds ratio, 4.7; 95% CI, 1.5 to 14.6; P = .002). No other statistically significant demographic and obstetric differences were found between the groups. There were no cases of perinatal death in the study population. Women treated for infertility had statistically significant higher rates of cesarean section compared with those who conceived spontaneously (71.4% versus 41.3%; odds ratio, 3.6; 95% CI, 1.4 to 9.2; P =.002). Stratified analysis (the Mantel-Haenszel technique) was used to control for possible confounders such as low birth weight, pathologic presentations, failed induction, nonprogressive labor, and nonreassuring fetal heart rate tracings. None of those variables explained the higher incidence of cesarean section in the group treated for infertility.'] Is infertility treatment an independent risk factor for cesarean section among nulliparous women aged 40 and above?