The study was conducted in in San Francisco by the Kaiser Permanente Division of Research and is to be published this week in the American Journal of Obstetrics and Gynecology.
The elevated miscarriage risk appears to be due to caffeine and not coffee alone because caffeine from other sources like tea, hot chocolate, and caffeinated soda or fizzy drinks also raised the risk.
Dr De-Kun Li, who is a Research Scientist at the Division of Research, Kaiser Permanente, Northern California, led the investigation. Kaiser Permanente is the largest insurance group in the US. Li's co-authors were Xiaoping Weng and Roxana Odouli.
Some scientists have sugggested caffeine is harmful to the developing fetus because it crosses the placenta and the fetus's metabolic system is not sufficiently developed to cope with caffeine, which can affect cell development directly and indirectly through decreasing blood flow from the placenta.
Others are sceptical of the claim that caffeine increased miscarriage risk, and maintain that the women who do miscarry tend to reduce their caffeine intake anyway because of morning sickness which makes them averse to caffeine, and this leads to a false impression that reducing caffeine intake reduces miscarriage risk.
Li explained that while other studies have shown links between caffeine intake and miscarriage, he and his team were the first to thoroughly control for morning sickness and other factors.
"This study strengthens the association between caffeine and miscarriage risk because it removes speculation that the association was due to reduced caffeine intake by healthy pregnant women," said Li.
Li and colleagues studied the effect of caffeine in 1,063 pregnant Kaiser Permanente members who were in San Francisco from October 1996 to October 1998. None of the women changed their caffeine drinking pattern during pregnancy. They were asked questions in interview about the frequency, amount and types of drinks they had, and when they had them.
They were also asked about other potential risk factors such as: age, race, education, marital status, household income, smoking status, alcohol consumption, use of hot tub, exposure to magnetic fields while pregnant, and pregnancy-related symptoms such as nausea and vomiting. These were taken into account in the analysis.
Li and colleagues calculated the women's caffeine intake from coffee, tea, caffeinated soda and hot chocolate and monitored for pregnancy outcomes up to the 20th week of gestation.
For every 150 millilitres of drink, the researchers estimated the caffeine content to be: 100 mg in coffee, 2 mg in decaffeinated coffee, 39 mg in caffeinated tea, 15 mg in caffeinated soda or fizzy drinks, and 2 mg in hot chocolate.
The results showed that:
- 172 (16.2 per cent) women miscarried.
- 264 women (25 per cent) drank no caffeine at all while pregnant.
- 635 women (60 per cent) drank between 0 and 200 mg of caffeine per day while pregnant.
- 164 women (15 per cent) drank over 200 mg of caffeine per day.
- The women who drank over 200 mg of caffeine a day ran twice the risk of miscarriage compared to the group that had none.
- The women who had less than 200 mg a day still had a 40 per cent higher risk.
Li suggested that pregnant women should probably stop drinking caffeine during pregnancy because there is now strong evidence that it raises the risk of miscarriage.
Director of Women's Health at Kaiser Permanente in Northern California, Dr Tracy Flanagan, had this recommendation for pregnant women who feel a need for a daiy "energy jolt":
"Learn to perk up instead with natural energy boosts like a brisk walk, yoga stretches, snacking on dried fruits and nuts."
"If you definitely need caffeine to get you going, try keeping it to one cup or less a day. Avoiding it may be even better. Consider switching to decaffeinated coffee and other decaffeinated beverages during your pregnancy," she advised.
Click here for American Journal of Obstetrics and Gynecology.
Source: Kaiser Permanente.