A recent study revealed that pregnant women who consume caffeinated drinks equal to two or more cups of regular coffee a day double their risk to have a miscarriage.

The study, published today in the American Journal of Obstetrics and Gynecology, is the first to interview women about their caffeine habits before they have actually suffered a miscarriage. It is also one of the largest to look at the connection between caffeine and miscarriage. Caffeine has been mooted as a risk factor before, but studies have yielded conflicting results.

The study found that 200 mg of caffeine a day doubled the risk compared to women who abstained.

The study, conducted by the nonprofit Kaiser Permanente Division of Research, in Oakland, California, analyzed caffeine habits of 1,063 women. About 12.5 percent of women reported no caffeine consumption during the first 20 weeks of pregnancy miscarried versus 25.5 percent of women consuming more than 200 milligrams of caffeine per day. Researchers discovered that 16 percent of the women had miscarriage.

The difference in risk was most noticeable in miscarriages that happened after eight weeks of pregnancy. The study did not analyze the effects of caffeine on pregnancies past 20 weeks.

"Women who are pregnant or are actively seeking to become pregnant should stop drinking coffee for three months or hopefully throughout pregnancy. There has been a lot of uncertainty about this. There was no firm advice from professional societies to say what a pregnant woman should do about caffeine intake," said De-Kun Li, a research scientist in Kaiser’s Northern California division and an author of the paper.

The risk was still high even when researchers took into account age, alcohol consumption, smoking and other factors related to miscarriage.

“The data has been consistent, but the question has been whether this is a real effect or not. Our study has addressed that issue. Hopefully that relationship is much more firmly established now,” said Li.

Based on the findings, Pat O’Brien, consultant obstetrician and spokesperson for the Royal College of Obstetricians and Gynecologists, said he would now recommend women in their first 12 weeks of pregnancy to abstain from caffeine alto together.

“The first 12 weeks is a very vulnerable time for the baby. It’s when most miscarriages occur,” he said. However, he said it was unclear whether pregnant women needed to avoid caffeine in later pregnancy.

Some doctors also have an explanation for why the miscarriages happen in the case of women who drink coffee during pregnancy. It seems that caffeine passes easily from the mother through the placenta and into the fetus, which is unable to metabolize the drug in a healthy way. Doctors think that later in pregnancy, as the baby’s metabolic system matures, caffeine is safe.

Despite the new findings, Dr. Aaron Caughey, a perinatologist at UCSF said it is important to note that the majority of miscarriages, up to 80 percent, happen due to chromosomal abnormalities that have nothing to do with the mother’s behavior. He also added that the last thing women who have had miscarriages need to do is blame themselves.

Li recommended zero dose of coffee during pregnancy. However, “if you really have to drink coffee, try to limit it to one cup or at the most two cups.” Pregnant women should switch to decaffeinated beverages, he added.

The study also discovered that women with nausea during pregnancy have a lower risk of miscarriage compared to women without nausea, Dr. Jorge Chavarro, research fellow at the Harvard School of Public Health said. That happens because women with nausea usually develop distaste for coffee.

An estimated one in five pregnancies in the UK will end in miscarriage, affecting around 250,000 women in the UK every year. Besides the latest discovery, there are also other risks, which lead to miscarriage such as increased maternal age, a previous history of miscarriage and infertility. Anyway, the causes of the majority of miscarriages are not fully understood.