Prenatal risk indicators of a prolonged pregnancy. The Danish Birth Cohort 1998-2001

Background. Few prenatal risk factors of prolonged pregnancy, a pregnancy of 42 weeks or more, are known. The objective was to examine whether sociodemographic, reproductive, toxicologic, or medical health conditions were associated with the risk of prolonged pregnancy. Methods. Data from the Danish Birth Cohort in Denmark were used. Interview data from 53,392 participants with live-born singleton deliveries in the period 1998-2001 were available at the time of this study. The participants were interviewed by telephone at 12 and 30 weeks' gestation, and 6 and 18 months after delivery. Statistical analyses were done using logistic regression. Results. Women with a pre-pregnancy body mass index of 25 kg/m(2) or more had a high risk of prolonged pregnancy. If the pre-pregnancy body mass index was 35 kg/m(2) or more the odds ratio was 1.52 (95% CI 1.28-1.82). Nulliparity also increased the risk of prolonged pregnancy (OR ( 95% CI) = 1.35 (1.27-1.44)). Conclusions. The risk of post-term delivery was high in women with a pre-pregnancy body mass index of 25 kg/m(2) or more, and in nulliparous women.