Background: Approximately 10 percent of all pregnancies continue to 42 weeks'gestation. The true prevalence of pregnancies lasting longer than 41 or 42 weeks'gestation has been difficult to establish because many such diagnoses occur as a result of an inability to accurately establish time of conception. Several studies indicate that menstrual dating is often inaccurate and can lead to induction of labor for a presumed but inaccurate diagnosis of postterm pregnancy. It is now widely accepted that a more precise method of determining gestational age is the first trimester measurement of crown-rump length of the fetus.

Methods: We carried out a cross- sectional study on patients who were referred for a prenatal care to Arash hospital, Medical Sciences/University of Tehran. Women in the first trimester of pregnancy who presented to the center during the study period with precise  date of last menstrual period were eligible to enroll. They underwent an ultrasound dating scan between 8 and 12 weeks, measuring crown-rump length. The estimated date of delivery (EDD) was changed if there was a discrepancy of more than 5 days from the gestation, calculated from the last menstrual period (LMP). For the remaining women (no-scan group), gestation was determined using the LMP. Data were analyzed using SPSS, 11. Statistical significance was assessed with Chi-square, student t-test and Anova, using a significance threshold of < 0.05.

Results: Of 217 women who were enrolled, 10.6% of the patients who underwent an ultrasound dating scan were postdate versus 3.7% of the patients whose gestational age calculated from LMP (p=0.005).

Conclusions: The application of first trimester sonographic measurement of crown-rump length of the fetus results in a significant reduction of the diagnosis of postterm pregnancies.