According to the Mayo Clinic, the average cup of American-style brewed coffee contains between 95 and 200 milligrams (mg) of caffeine.
Moreover, every 100 mg of caffeine consumed per day increased the length of pregnancy by five hours. And when the caffeine came from coffee (as opposed to other sources) the length of pregnancy was extended eight extra hours, the study authors found.
Given this finding, it is likely that it is not only the caffeine, but something else in coffee that is acting to extend pregnancy, the researchers added.
Dr. Jennifer Wu, an obstetrician/gynecologist at Lenox Hill Hospital in New York City, said, "Other studies have indicated that caffeine can affect fetal weight, so this is in accord with findings of other studies."
Why caffeine might cause this effect is unclear, she said.
"We do know that caffeine crosses the placenta and the baby is not able to metabolize it very well, [so] it may affect some of the factors associated with growth," Wu theorized.
She advised that women limit the amount of caffeine they consume during pregnancy. The World Health Organization says 300 mg a day, but in the United States the recommended amount is 200 mg a day, she added.
Wu noted that is the amount of caffeine in two small cups of coffee, not a "Starbucks size coffee." There is less caffeine in a cup of tea, or a piece of chocolate, which has about 35 mg of caffeine, she said.
For more information on low birth weight infants, visit the March of Dimes.
SOURCES: Verena Sengpiel, M.D., Ph.D., department of obstetrics and gynecology, Sahlgrenska Academy, Sahlgrenska University, Goteborg, Sweden; Jennifer Wu, M.D., obstetrician/gynecologist, Lenox Hill Hospital,
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