The diagnosis of postterm pregnancy rests on establishment of the correct gestational age.
The first step in management of a patient with suspected postterm pregnancy is a careful review of the criteria used to establish the gestational age. The most common infor-mation used to determine gestational age include the pa-tient’s reported LMP and the first trimester ultrasound.
Ultrasound is most accurate for determining dating for gestational age when it is performed from 6 to 12 weeks of gestation. If the patient’s LMP predicts an estimated date of delivery (EDD) that is within 10 days of an EDD determined by an ultrasound performed between 12 and 20 weeks of gestation, then the gestational age is consid-ered fairly accurate. Once the EDD is determined, it should not be changed unless more accurate information is disclosed.
With improved access to prenatal care and greater importance placed on accurate gestational age assessment, the percentage of patients in whom postterm pregnancy is suspected has diminished. Nonetheless, a substantial number of patients do not seek prenatal care early in preg-nancy or do not have an accurate gestational age deter-mination. The prevalence of postterm pregnancy varies regionally, depending on the use of first-trimester ultra-sound for gestational dating and routine labor induction.