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June 30, 2000 -- It is a simple date, but it is one of the most important pieces of information you can give your gynecologist if you are pregnant: the first day of your last menstrual period. It's from this date that your doctor will estimate when you became pregnant and plan your care accordingly.
"The more accurately we can date a pregnancy, the better the outcome should be," says Steven R. Goldstein, MD, professor of obstetrics and gynecology at NYU School of Medicine in New York City. "If you're intending to get pregnant, keep good menstrual records and take a home pregnancy test."
He notes that in many cases, an ultrasound can help estimate the baby's gestational age, but says, "an ultrasound to determine a mother's due date will be more accurate the earlier it's done."
The validity of knowing the first day of your last period seems to be reinforced by a Danish study published in the June issue of the journal Obstetrics-Gynecology. The authors found that when women are unable to provide this information, their pregnancy is more likely to have a bad outcome.
However, women in the study who were unsure of their last period's date tended to be younger and have closely spaced pregnancies, according to study authors Tri Huu Nguyen, MD, and colleagues of the ultrasound department at Herlev Hospital in Herlev, Denmark.
"First of all, these are unintended pregnancies," Donnica Moore, MD, tells WebMD about the study participants. "The mothers with unreliable dates were significantly younger, or had had 'back-to-back' pregnancies, in which poor outcomes are more common." Moore is the president of Sapphire Women's Health Group, a consulting and medical education group based on Branchburg, N.J. She also has been a medical commentator for NBC's "Later Today" show.
For more information from WebMD, visit our Diseases and Conditions Pregnancy page.