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Obstetricians usually date pregnancies in weeks, from the first day of the last menstrual period.
Obstetricians usually date pregnancies in weeks, typically from the first day of the last menstrual period. Since fertilization occurs 2 weeks after this date, after a new ovum is released, obstetric dates are about 2 weeks longer than embryologic dates.
Pregnancy is considered to last 266 days from the time of conception, or 280 days (266 + 14) from the first day of the last menstrual period (if menses are regular at 28 days).
Naegele's rule states that the estimated date of confinement (EDC) can be approximated as:
EDC = (first day of the last menstrual period) - (3 months) + 7 days1
Pregnant patients are given a "due date." This is the layperson or generic term for what we know as the EDC or EDD. But the "due date" can serve as a source of tremendous anxiety as patients cross this day without giving birth and consider themselves "late," which most equate with abnormal. If a patient goes into labor after 40 weeks (but before 42 weeks and is still not postterm) and there is any problem, it sometimes makes us look bad in that we "allowed" her pregnancy to be "late," when we did not do anything wrong. Why do we do this? No one else gets roped into such finality. Weather forecasters aren't held to telling us what exact time, to the minute, that it will start to rain. They just say that it will likely begin to rain sometime this afternoon. Airlines tell us the exact minute planes will land, but no one takes that seriously. It's cruel and nonproductive to give individuals a due date at 40 weeks. I try to explain it this way to my patients . . .
Explaining what the due date really means
I tell a patient that the term "due date" is actually the lay term for what physicians call the "estimated day of confinement." I then allow her to come to the realization that an estimate is not precise and little more than a "guesstimation." I then tell her that it is unfair for us to tell her exactly what day her baby will be born when we don't really know or see it that way. That's as ridiculous as a pediatrician being able to tell a mother the exact day her baby is due to talk or walk. Instead, parents are given a "range of time within which it is normal for that to occur." In obstetrics we have a similar concept for the end of pregnancy, called "term." That is really the "due time period" in which it is most likely that someone will go into labor. It is from 37 to 42 weeks.
The "due date" at 40 weeks is simply a midpoint in that time period. It is not really "late" or postterm until reaching 42 weeks of pregnancy. If a patient's "due date" is the 15th of the month, I suggest a patient tell her family and friends that she isn't "due" to deliver until the end of the month (at 42 weeks). I then tell her that there was an obstetrician in our community who used to give patients 42 weeks as their "due date." As you might expect most of his patients delivered before that day. He got the very popular reputation as the doctor to go to in town if you wanted to deliver "early" and what patient doesn't want that-rather than delivering "late."
Avoiding "postdate" fetal testing