Background: Recent data suggest that the chance of successfully maintaining a pregnancy may be influenced by the sex of previously born children. We explored a possible relation between sex of the first-born infant and the risk of preterm birth in the second pregnancy.
Methods: Using data from the National Medical Birth Registries in Denmark 1980–2004 and Sweden 1980–2001, we selected all women whose first and second births were singleton and who had information on sex of first-born infant and gestational age for the second (Denmark, n = 393,686; Sweden, n = 603,282). Cox proportional hazards regression analysis was used to estimate the hazard ratio of preterm birth in the second pregnancy according to the sex of the first-born infant.
Results: Compared with women whose first baby was a girl, women with boys had an increased risk of preterm birth in a second pregnancy (hazard ratio = 1.10 [95% confidence interval = 1.07–1.13]). This result was consistent in the 2 populations. The association was not confounded by maternal age, interpregnancy interval, or sex of the second infant or by maternal characteristics that do not vary from one pregnancy to the next.
Conclusions: Exposure to a male fetus may increase a woman's risk of preterm delivery in the next pregnancy. While the findings have no direct public health relevance, they may suggest new pathways by which preterm birth can occur.