Background: Delivery of a boy has been reported to increase a woman's risk of recurrent miscarriage in subsequent pregnancies. We explored whether delivery of boys similarly increases the risk of a subsequent stillbirth.
Methods: We identified all Danish women delivering their first child (singleton) between 1980 and 1998 (n = 499,731) using the Danish Birth Registry. These women had subsequent singleton births through 2004 (n = 558,314). We assessed the risk of stillbirth conditional on sex of prior children.
Results: The risk of stillbirth was increased by 12% after deliver of boys compared with girls (relative risk = 1.12 [95% confidence interval = 1.02–1.23]). This association did not appear to be explained by maternal confounders.
Conclusion: Stillbirth risk appears to be slightly higher among the pregnancies of women who have previously delivered a boy. One possible mechanism is maternal immune response to male-specific minor histocompatibility antigens initiated during pregnancies with boys.
From the aThe National Institute of Public Health, University of Southern Denmark, Copenhagen, Denmark; bThe Fertility Clinic, and cDepartment of Neonatology, Copenhagen University Hospital, Rigshospitalet, Copenhagen, Denmark; and dDivision of Epidemiology, University of Southern Denmark, Odense, Denmark.
Submitted 14 December 2008; accepted 3 March 2009; posted 9 November 2009.
Supported by a research grant from the Copenhagen University Hospital, Rigshospitalet (to H.S.N.).
Correspondence: Henriette S. Nielsen, The Fertility Clinic, Copenhagen University Hospital, Rigshospitalet, DK-2100 Copenhagen Ø, Denmark. E-mail: firstname.lastname@example.org.