As few as three alcoholic drink a weeks could make it more difficult for a woman to become pregnant, particularly if the drinking occurs in the second half of her menstrual cycle. The study, published this week in Human Reproduction, investigated alcohol intake and the probability of conceiving – it is the first study to evaluate the association during specific phases of the menstrual cycle.
University of Louisville School of Public Health and Information Sciences researcher Kira Taylor and her team analyzed data from more than 400 women for up to 19 months. The women completed daily diaries reporting how much alcohol they drank and what type.
Heavy drinking was defined as more than six alcoholic drinks a week, moderate drinking was three to six drinks a week, and binge drinking was defined as four or more drinks on a single day. A drink was defined as a third of a liter of beer, a medium glass of wine or just under a double shot of spirits. The researchers collected information on factors that could affect the results, such as age, medical history, smoking, obesity, intercourse frequency, use of birth control methods and intention to become pregnant.
“We found that heavy drinking during any phase of the menstrual cycle was significantly associated with a reduced probability of conception compared to non-drinkers,” Taylor said. “This is important because some women who are trying to conceive might believe it is ‘safe’ to drink during certain parts of the menstrual cycle.”
The research also found during the luteal phase, or the last two weeks of the menstrual cycle before bleeding starts and when the process of implantation occurs, not only heavy drinking but also moderate drinking was significantly associated with a reduced probability of conception. At the time of ovulation, usually around day 14 of the cycle, consuming a lot of alcohol – either heavy or binge drinking – was significantly associated with reduced chances of conception.
“If we assume that a typical, healthy, non-drinking woman in the general population who is trying to conceive has approximately a 25% chance of conceiving during one menstrual cycle, then out of 100 women approximately 25 non-drinkers would conceive in a particular cycle, about 20 moderate drinkers would conceive and only about 11 heavy drinkers would conceive,” Taylor said. “But the effect of moderate drinking during the luteal phase is more pronounced and only about 16 moderate drinkers would conceive.
“Our study only included a few hundred women and, while we believe the results strongly suggest that heavy and even moderate alcohol intake affects the ability to conceive, the exact percentages and numbers should be viewed as rough estimates,” she said.
Each extra day of binge drinking was associated with an approximate 19% reduction in the odds of conceiving during the luteal phase and a 41% reduction during the ovulatory phase. The researchers found no difference in their results between different types of drinks. The influence of drinking by male partners was not assessed.
While the study does not show that drinking alcohol causes the reduction in the chances of becoming pregnant, it does point to an association. Possible biological mechanisms to explain the association could be that alcohol intake affects the processes involved in ovulation so that no egg is released during the ovulatory part of the cycle, and alcohol could affect the ability of a fertilized egg to implant in the womb.
“The results in this study should not be construed to mean that drinking alcohol prevents pregnancy. In other words, alcohol is not birth control. Even if a woman drinks alcohol heavily, if she has unprotected intercourse, she can become pregnant,” Taylor said.