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Birthdays are arriving a little earlier than they used to, and that's not always a good thing.
In 1992, the most common gestational age for single live births was 40 weeks, according to a new study by March of Dimes scientists. By 2002, that number had dropped to 39 weeks.
In addition, the percentage of babies born between 34 and 36 weeks increased. The study appears in a supplement to the current issue of the journal Seminars in Perinatology.
Traditionally, doctors have estimated women's due dates at 280 days, or 40 weeks, after the first day of their last menstrual period. Study co-author Nancy Green, medical director for the March of Dimes, says she believes the shift in gestational age is mainly a result of rising rates of labor inductions and scheduled C-sections.
"We are in an era of increasing intervention," Green says. "Much of that is good. Mortality rates have gone down for infants."
The American College of Obstetricians and Gynecologists recommends that labor not be induced before the 39th week of pregnancy, Green says. Babies born anytime between 37 and 41 weeks are considered full-term, so delivering at 39 weeks instead of 40 weeks isn't a problem -- as long as it really is 39 weeks, she says.
"Except for a few newer methods, estimating pregnancy length can be off by as much as two weeks," she says. Only women who have had first trimester ultrasound or conceived with the help of artificial reproductive technology -- in vitro fertilization -- can be certain of their due date, Green notes.
So a woman who schedules labor induction or a C-section for the 37th or 38th week of her pregnancy might actually end up delivering a baby whose gestational age is only 35 or 36 weeks -- in other words, preterm. In 2002, 7.7% of all single births were at 34 to 36 weeks gestation, compared with 6.9% in 1992, according to the March of Dimes study.
And what a difference a few weeks can make. "Those babies don't do as well as the full-term," Green says.
Compared with full-term babies, those born at 34 to 36 weeks are more likely to have such problems as respiratory distress syndrome, low blood sugar and low body temperature.
"Those babies all do well, but they spend an extra couple of days or a week in the hospital," says Peter Bernstein, an obstetrician/gynecologist at Albert Einstein College of Medicine and Montefiore Medical Center in New York. Bernstein was not involved in the study.
Green and her co-authors found that gestational age decreased from 1992 to 2002 even for deliveries that were not scheduled.
"Do I think the species is evolving? I don't think so." Instead, Green says, she suspects that many births recorded as spontaneous actually were induced.
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