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Objective: To examine possible reasons why a male fetus constitutes a risk factor for preterm delivery. Study design: Retrospective study of deliveries from hospital database in a UK teaching hospital. The population comprised all deliveries >23 weeks over an 11-year period, excluding multiples, terminations and pregnancies with major abnormalities including indeterminate gender. Obstetric variables and outcomes were initially compared in male and female babies for preterm births in different gestation bands, extreme (<28 weeks), severe (29-32 weeks) and moderate (33-36 weeks). For each, the odds ratios with 95% confidence intervals for preterm delivery were calculated. Then, using binary logistic regression with adjusted odds ratios with 95% confidence intervals, putative causal pathways that might explain the male excess were tested. Results: 75,725 deliveries occurred, of which 4003 (5.3%) were preterm. Males delivered preterm more frequently (OR 1.13, 95% CI 1.06-1.20). This was due to spontaneous (OR 1.30, 95% CI 1.19-1.42) but not iatrogenic (OR 0.96, 95% CI 0.87-1.05) preterm birth. There was an increased risk of pre eclampsia among preterm females. Although males were larger, and male pregnancies were more frequently nulliparous and affected by some other obstetric complications (abruption, urinary tract infection), these did not account for their increased risk. Any effect of growth restriction could not be properly determined. Conclusions: Being male carries an increased risk of spontaneous but not iatrogenic preterm birth. The reasons behind this remain obscure. © 2008 Elsevier Ireland Ltd. All rights reserved.

Original publication




Journal article


European Journal of Obstetrics Gynecology and Reproductive Biology

Publication Date





123 - 126