Adverse pregnancy outcomes and familial confounding in asthmatic women – A population based family design study

Abstract

Background Almost 10 % of women in childbearing age have asthma which is associated with several adverse pregnancy and perinatal outcomes. Familial factors may confound these associations. By conducting a family study with differently exposed siblings and cousins, unmeasured shared factors - genetic and environmental – could be evaluated. Methods Data on all women in Sweden with singleton births from 2001 to 2013, prescribed drugs and physician diagnosed asthma was retrieved from several nationwide registers. Logistic and linear regression estimated the association between maternal asthma and studied outcomes. In differently exposed pregnant siblings and cousins conditional logistic and linear regression were used. Results In total, 661,956 women with eligible pregnancies were included and 10.2 % of the study population had asthma. We identified 288,454 cousins and 199,584 sisters giving birth during the study period. Women with asthma had increased risks for preeclampsia (aOR 1.17 (95% CI 1.12 – 1.21)), placental abruption (aOR 1.36 (95% CI 1.20 – 1.54)), instrumental vaginal delivery (aOR 1.11 (95% CI 1.07-1.14)), elective caesarean section (aOR 1.33 (95% CI 1.29-1.38)), emergency caesarean section (aOR 1.26 (95% CI 1.22-1.29)) and small for gestational age (aOR 1.21; (95% CI 1.15 – 1.28)). In the conditional regression analyses, controlling for familial factors, the associations remained for studied outcomes (eg. preeclampsia aOR siblings 1.18 and aOR cousins 1.24). Conclusion Familial (genetic and environmental) factors shared by siblings and cousins does not seem to explain observed association between maternal asthma and adverse pregnancy outcomes.