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Expectant moms take longer than expected.

Expectant moms take longer than expected

For many women, the news of pregnancy instantly sparks the question: "When is the baby due?" Typically, the obstetrician consults a standard algorithm to formulate an answer. And most likely, the baby will arrive about a week behind schedule.

That's the conclusion -- at least for middle-class whites -- of Boston researchers who measured normal pregnancy durations and compared them with traditional gestation estimates.

Led by obstetrician Robert L. Mittendorf of the Harvard School of Public Health, the team examined records from a private practice, retrospectively evaluating all 339 pregnancies of white, middle-class patients who gave birth between April 1, 1983, and March 31, 1984. They excluded patients lacking precise records of normal menstrual cycles, as well as pregnancies complicated by illness, induced labor or cesarean section. Using menstrual histories to determine ovulation dates, they measured the gestations of the 114 remaining women.

In the June OBSTETRICS & GYNECOLOGY, the team reports a median gestation of 274 days for the 31 first-time mothers and 269 days for the 83 women who had previously given birth. That's eight and three days longer, respectively, than the standard estimate of gestation.

Since ancient Rome, physicians have assumed human gestation lasted 266 days, based on 9-1/2 menstrual cycles. Now known as Naegele's rule, this remains the standard by which most U.S. obstetricians predict due dates, Mittendorf says.

"What we're saying is maybe the real due date for whites is a week later, and this will almost certainly have implications for the post-term infant," he asserts. "For example, if a patient is two weeks beyond Naegele's rule for a due date, [the obstetrician] might induce labor for post-maturity" without realizing the infant is actually only a week overdue.

Peter S. Heyl of Boston's Beth Israel Hospital expresses some skepticism of the limited sample size but regards the Harvard findings as potentially very significant. "In my line of work, gestational age means a lot. It does make a difference when physicians will induce [labor in] somebody based entirely on the fact that she's reached 42 weeks." Induced labor has a higher incidence of infection, says Heyl, who adds: "I think it's pretty safe to say that an induced labor is more likely to end in a cesarean section than a spontaneous labor."

Mittendorf is now evaluating data on more than 10,000 women in hopes of further redefining gestation to account for race, age, sex of baby, and the mother's history of cigarette, alcohol and drug use.
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Author:Stolzenburg, W.
Publication:Science News
Date:Jun 16, 1990
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