Just two cups of coffee a day doubles a pregnant woman's risk of miscarriage, new research shows.
In what is believed the clearest and strongest evidence yet linking caffeine to miscarriage, California researchers who followed more than 1,000 pregnant women found those who consumed 200 mg or more of caffeine daily -- the equivalent of two cups of regular coffee, or five, 12-ounce cans of caffeinated pop -- had twice the risk of miscarriage as women who avoided caffeine.
Even women who consumed less that 200 mg of caffeine a day had a 42% increased risk of losing the pregnancy.
"We really don't know much about the risk factors for miscarriage other than a few commonly known ones, like age, chromosomal stuff and smoking," says lead investigator
Dr. De-Kun Li, a reproductive and perinatal epidemiologist at the Kaiser Permanente Division of Research in Oakland, Calif.
Past studies have shown a link between caffeine and miscarriage, but there has been a complicating factor: Women who get morning sickness often give up coffee. And these women are less likely to have a miscarriage in the first place, as morning sickness is one sign of a healthy pregnancy.
One theory is that morning sickness, which can actually occur at any time of the day, is nature's way of protecting the unborn baby from toxins and-chemicals that can harm fetal development.
Dr. Li's study is the first to "control" for morning sickness, meaning scientists took it into account before tallying up the final results. About 15% of "clinically recognized" pregnancies end in miscarriage.
Many women miscarry before they know they are pregnant. The causes for most miscarriages are not wholly understand and many suspected risk factors remain controversial or unproven.
Caffeine is among the most frequently ingested pharmacologically active substances in the world, according to the new study, published today in the online issue of the American Journal of Obstetrics and Gynecology.
Caffeine readily crosses the placental barrier, so it can go straight to the fetus. The earlier the exposure, the less likely the fetus is to metabolize it because of an under-developed metabolic system. As well, caffeine, particularly at high doses, makes blood vessels contract.
If that contraction is severe enough, it may reduce blood flow to the placenta. "That can have detrimental effects to the fetus as well," Dr. Li said.
His team recruited 1,063 San Francisco women from October of 1996 to October of 1998. Women were recruited early in their pregnancy -- as early as four weeks gestation -- "so we were able to capture a lot of miscarriages that most studies missed," Dr. Li said.
The expectant mothers were asked about their caffeine consumption and their changing pattern of drinking. They were also asked about nausea and vomiting and other factors that might influence how much caffeine they consumed.
Overall, 172 women, or 16%, miscarried. The higher the intake of caffeine from any source -- coffee, tea, caffeine-containing soft drinks or hot chocolate -- the greater the risk of early or late miscarriage.Compared with non-users, women who consumed 0-200 mg of caffeine daily had an increased risk of miscarriage (15% versus 12%). The risk was higher (25%) in women who consumed more than 200 mg of caffeine daily.
The researchers estimated the amount of caffeine intake in beverages using the following conversion: For every 150 millilitres of beverage, 100 mg for caffeinated coffee, two mg for decaffeinated coffee, 39 mg for caffeinated tea, 15 mg for caffeinated soda and two mg for hot chocolate.
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