Pregnancy risk increases from 41 weeks of gestation

The aim of this study was to evaluate the mortality and morbidity of conservatively managed post-term pregnancies (gestation 294 days and beyond).

This is a population-based prospective study. The sample was comprised of all women (N=17,493) with a singleton pregnancy in one Norwegian county from 1989 to 1999, with a second-trimester ultrasound examination and delivery after 37 completed gestational weeks.

One thousand three hundred and thirty-six (7.6%) of the deliveries were post-term. In this group, the increase in perinatal mortality reached borderline significance [relative risk (RR) 2.0; 95% confidence interval 0.9-4.6]. Perinatal morbidity expressed as Apgar score <7 at 5 min (RR 2.0; 95% confidence interval 1.2-3.3), and transferal to neonatal intensive care unit (RR 1.6; 95% confidence interval 1.3-2.0) were significantly more frequent. However, RR for perinatal death calculated per 1000 ongoing pregnancies increased significantly from 0.2 in week 37-3.7 in week 42, using perinatal mortality in gestational week 41 as a reference.

Our results indicate that expectant management of post-term pregnancies allowing pregnancies to continue up to week 43 carries a risk for perinatal mortality and morbidity. The risk increases already from gestational week 41. The guidelines for management of post-term pregnancies should be revised.