Early termination of pregnancy: Differences in gestational age estimation using last menstrual period and ultrasound in Mexico

Background: Gestational age estimation is key to the provision of abortion, to ensure safety and successful termination of pregnancy. We compared gestational age based on reported last menstrual period and ultrasonography among a large sample of women in Mexico City's public first trimester abortion program, Interrupcion Legal de Embarazo (ILE). Methods: We conducted a retrospective study of 43,219 clinical records of women seeking abortion services in the public abortion program from 2007 to 2015. We extracted gestational age estimates in days based on last menstrual period and ultrasonography. We calculated the proportion of under- and over-estimation of gestational age based on last menstrual period versus ultrasonography. We compared overall differences in estimates and focused on discrepancies at two relevant cut-offs points (70 days for medication abortion eligibility and 90 days for ILE program eligibility). Results: On average, ultrasonography estimation was nearly 1 (- 0.97) days less than the last menstrual period estimation (SD = 13.9), indicating women tended to overestimate the duration of their pregnancy based on recall of date of last menstrual period. Overall, 51.4% of women overestimated and 38.5% underestimated their gestations based on last menstrual period. Using a 70-day limit, 93.8% of women who were eligible for medication abortion based on ultrasonography would have been correctly classified using last menstrual period estimation alone. Using the 90-day limit for ILE program eligibility, 96.0% would have been eligible for first trimester abortion based on last menstrual period estimation alone. Conclusions: The majority of women can estimate gestational age using last menstrual period date. Where available, ultrasonography can be used, but it should not be a barrier to providing care.