Accuracy of gestational age estimated by menstrual dating in women seeking abortion beyond nine weeks

Abstract

Objective: We sought to quantify the accuracy of estimating gestational age by reported last menstrual period among women seeking surgical abortion. We observed that women seeking surgical abortion underestimated their gestational age when making the appointment, leading to poor allocation of resources. This tendency to underestimate has not previously been reported and differs from the accurate dating reported among women choosing either medical abortion or continuation of the pregnancy.

Methods: We performed a retrospective review of randomly selected medical records for women with abortions scheduled at 9 to 20 weeks' gestation (n = 415) at two clinics in Vancouver between 2002 and 2008.

Results: The mean gestational age calculated by menstrual dates (14.3, SD 3.9) was 1.2 (95% CI 0.9 to 1.4) weeks less than that calculated by ultrasound (15.5, SD 3.4) (P < 0.001). Greater gestational age was associated with a larger discrepancy (r = 0.192, P < 0.001). Variables other than gestational age (maternal age, parity, previous abortions, illicit drug use, and contraceptive method at conception) were not significant predictors of inaccurate menstrual dating.

Conclusion: Women seeking surgical abortion for pregnancies of 9 to 20 weeks underreport gestational age by an average of 1.2 weeks using menstrual dating. We found that women who intended to continue with their pregnancy overestimated their gestational age, those seeking very early abortion estimated most accurately, and those seeking surgical abortion at more than nine weeks had a clinically significant underestimation of their gestational age. Clinicians referring and counselling women who are considering surgical abortion must facilitate timely access to clinical or ultrasound dating of their pregnancy.