A comparison of recalled date of last menstrual period with prospectively recorded dates.

BACKGROUND: Women are often asked to recall the first day of their last menstrual period (LMP date) in a clinic setting (i.e., pregnancy dating, x-rays). There are no data supporting the validity of these reports.
METHODS: Using data from a group of 385 women ages 35-49 from a larger cohort study in the Washington, DC, area, we constructed menstrual segments from a prospective daily menstrual record. We then compared the first day of a menstrual segment to a woman's recalled LMP date at a subsequent study-related clinic appointment to assess the accuracy of recall.
RESULTS: More than half of the women (56%) accurately recalled their LMP date; 74% were within 1 day, and 81% were within 2 days. Women tended to underreport (25%) the length of time since their last menstrual period rather than overreport the length of time (19%). Recall accuracy did not vary significantly with education or by whether the woman usually recorded her menstrual cycle when not in the study. As one might expect, women with a shorter recall duration tended to report more accurately. DISCUSSION: Women appear to recall their LMP dates fairly accurately, but inaccurate recall was not random. When length of recall was 3 weeks or longer, women tended to overestimate the time since LMP. This suggests that gestational age calculated from LMP date will tend to be overestimated. Most women can recall the date of their LMP reasonably well regardless of their education and whether they usually record their LMP dates.