The study was conducted by researchers at Kaiser Permanente Division of Research and was led by De-Kun Li, MD, Ph.D..
The research is significant as it controls for the first time pregnancy-related symptoms of nausea, vomiting and caffeine aversion that till now had interfered with researchers finding how caffeine really affects the risk of miscarriage.
"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," Dr Li said.
As a part of the study 1,063 pregnant Kaiser Permanente members in San Francisco were analysed from October 1996 through October 1998 with researchers analysing how caffeine affected women who never changed their pattern of caffeine consumption during their pregnancy.
The researchers found that consuming two or more cups of regular coffee or five 12-ounce cans of caffeinated soda a day was enough to double the risk of miscarriage.
Women who consumed less than 200 mg of caffeine daily had more than 40 percent increased risk of miscarriage.
The increased risk of miscarriage appeared to be due to the caffeine itself, rather than other possible chemicals in coffee because caffeine intake from non-coffee sources such as caffeinated soda, tea and hot chocolate showed a similar increased risk of miscarriage.
"The main message for pregnant women from these findings is that they probably should consider stopping caffeine consumption during pregnancy because this research provides clearer and stronger evidence that high doses of caffeine intake during pregnancy can increase the risk of miscarriage," said Li.
Caffeine crosses through the placenta to the foetus, but can be difficult for the foetus to metabolise because of the under-developed metabolic system. It may also influence cell development and decrease placental blood flow, which may lead to an adverse effect on foetal development.
Appearing in the current online issue of American Journal of Obstetrics and Gynecology, the study included Xiaoping Weng, Ph.D. and Roxana Odouli, MSPH, also with the Kaiser Permanente Division of Research. It was supported in part by the California Public Health Foundation.