The timely onset of labor and delivery is an important determinant of perinatal outcome. Both preterm and postterm births are associated with higher rates of perinatal morbidity and mortality than pregnancies delivering at term.
Postterm pregnancy refers to a pregnancy that has ≥420/7ths weeks of gestation or ≥294 days from the first day of the last menstrual period . Accurate pregnancy dating is critical to the diagnosis.
In the United States, approximately 27 percent of pregnancies deliver in the 40th and 41st week and 5.5 percent deliver at ≥42 weeks . A study of postterm birth rates in 13 European countries observed a wide range across the continent: from 0.4 percent in Austria and Belgium to 8.1 percent in Denmark . The authors attributed the variation to differences in prenatal assessment of gestational age and obstetric practices.
The prevalence of postterm pregnancy in a population is affected by several factors. One of the most important factors is whether routine early ultrasound assessment of gestational age is performed. Among pregnancies dated by first trimester ultrasound examination, the prevalence of delivery ≥42 weeks is about 2 percent (versus 6 to 12 percent by last menstrual period [LMP]) [4-7] and the prevalence of delivery ≥41 weeks ranges from 5 to 11 percent (versus 13 to 22 percent by LMP) [6,8-10]. In a meta-analysis that compared the rate of labor induction for postterm pregnancy in women who underwent sonographic estimation of their delivery date (EDD) in early pregnancy with the rate in women whose EDD was calculated from their LMP, early routine ultrasound examination reduced the rate of intervention for postterm pregnancy (OR 0.68, 95% CI 0.57-0.82) .
Other factors that affect the prevalence of postterm pregnancy in a population include the rate of spontaneous preterm birth, the prevalence of primigravid women (who are more likely to deliver postterm), and the prevalence of women with pregnancy complications (who are less likely to deliver postterm). Local practice patterns, such as the rate of scheduled cesarean delivery and elective labor induction, will also affect the prevalence of postterm birth.