Women trying to get pregnant may be able to improve their chances by cutting back on the coffee, a study suggests.
The Johns Hopkins University study found that women who consume more than 300 milligrams of caffeine a day - roughly three cups of coffee or eight sodas - reduce their monthly chance of conception by 26 percent.
No effect was seen in women consuming less than 300 milligrams.
“On average, a couple has a 20 percent chance of getting a viable pregnancy each month,” said Dr. Ronald Gray, one of the researchers at the Johns Hopkins School of Public Health.
“The high caffeine drinkers would reduce that by a quarter. It would be around 15 percent.”
The study was published in the latest issue of the American Journal of Epidemiology.
Several previous studies also have indicated a link between caffeine and lower fertility.
Pregnant women already are warned to avoid or cut back on caffeine because there is some biological evidence that caffeine disrupts the menstrual cycle and may lead to early pregnancy loss.
A cup of coffee contains about 100 milligrams of caffeine, a cup of tea 50 milligrams, and a can of some kinds of soda 40 milligrams.
The study used data from interviews with 1,430 women conducted in 1989 and 1990, involving more than 2,000 pregnancies that occurred between 1980 and 1990.
About 8 percent of the women who neither smoked nor consumed large amounts of caffeine took a year or more to conceive, compared with 19 percent of women who consumed more than 300 milligrams of caffeine a day, the study found.
Regardless of the amount of caffeine they took in each day, smokers were found to be 15 percent to 20 percent less likely to become pregnant in a given month than nonsmokers.
It is believed that smoking increases how quickly caffeine is metabolized, which may decrease its effects.