Risk of recurrence of prolonged pregnancy.

We obtained data recorded in the Danish medical birth registry on all women with a prolonged pregnancy in the first delivery and in a subsequent delivery, during 1980-94, and a 5% sample of all women with two or more pregnancies recorded in the period 1980-92 (only the first two deliveries were used for analysis). The information on gestational age in the registry was obtained from birth records that had been completed by midwives. Data on social status and municipality of residence stem from various population registers. The post-term cohort included all women who gave birth to a singleton live infant recorded as post-term (gestational age 42 weeks or more) and a subsequent live infant. The term cohort included all women who had a first singleton live infant born at term (that is, gestational age 37-41 weeks) and a subsequent live infant. After excluding cases with missing information on gestational age (393 post-term cohort, 635 term cohort), paternity (184, 0), and stillborns (300, 177), we had 21 746 sibling pairs in the post-term cohort and 7009 sibling pairs in the term cohort. We studied the effect of changes in potential risk factors between pregnancies on post-term delivery with the computerised square dance design, which is based on Mackie’s concept of causation. The basic idea is to study the effect of removing possible component causes from a causal field among the participants exposed to the variable being studied. The design is based on internal comparison within the two cohorts. The risk of recurrence of post-term delivery was 19.9%. It increased with increasing gestational age in the index birth to approximately 30% for a gestational age of 44 weeks. In the term cohort 7.7% were post-term in the subsequent birth. The risk of recurrence of post-term delivery was reduced to 15.4% when the first and second child had different fathers (0.73, 0.63 to 0.84) (table). Having a different father did not affect the risk of post-term delivery of the second child in the term cohort (0.99, 0.69 to 1.41). A change in municipality and social status had no impact on the risk of recurrence of prolonged pregnancy. Length of pregnancy differed by more than one week in women who had changed partners, compared with those who had not (odds ratios 1.26, 1.13 to 1.41 in post-term cohort and 1.27, 1.04 to 1.54 in term cohort).