In fact, the study shows that women who do not drink at all can actually take longer to become pregnant than those women who consume an average of between a half and 14 alcoholic drinks a week.
The study of nearly 40,000 women in Denmark is large enough to offer reassuring evidence which contradicts the findings of previous, smaller studies suggesting that even low levels of alcohol intake are associated with longer waiting times to pregnancy. In women who had never given birth before, neither moderate nor high alcohol intake was related to a longer waiting time to pregnancy.
However Mette Juhl, the Danish researcher who headed the study, sounded a cautionary note: "Our research does not mean that consuming alcohol is better for fertility than not drinking alcohol at all. It is known from other studies that non-drinkers differ from moderate drinkers in many other aspects; for instance they may have a weaker constitution or other health problems which can influence fertility, directly or indirectly," she said.
Ms Juhl and her colleagues, from centres in Copenhagen and Aarhus, did find that women who had given birth before and who consumed more than the Danish recommended maximum weekly intake of 14 alcoholic drinks did have to wait longer to get pregnant. However, the evidence from the study was not strong for this as there were few women in this group. Older women and women who smoked or who were over-weight also had to wait longer to become pregnant. Non-drinking, over-weight women had to wait the longest of all. These findings support the evidence of previous studies.
The study was carried out within the Danish National Birth Cohort, a nationwide study of pregnant women and their babies. The researchers asked 39,612 women between 1997 and 2000 how much alcohol they had drunk weekly on average prior to conception and how long it had taken them to become pregnant. Low levels of drinking were measured as being between a half and two drinks a week, moderate drinking was between two and a half and 14 drinks a week, and high was measured as over 14 drinks a week.
Out of the 39,612 pregnancies, 29,844 were planned and did not have complicating factors such as endometriosis, ovarian or cervical cancer. Of these 29,844 women, roughly half became pregnant within two months of trying, but 15% had to wait more than a year. Most women (79%) reported an alcohol intake of between a half and seven drinks a week, 12% reported no intake at all and one per cent reported a high intake.
Of the women with a high alcohol intake, 22% had to wait more than a year to become pregnant, while only 14% in the low-drinking group had to wait that long. In the group of women who drank no alcohol 18% had to wait more than a year.
The researchers adjusted their results to take account of confounding factors such as pelvic disease, a history of fertility treatment, partly-planned pregnancies, age, smoking, the women's weight and whether they had given birth before or not.
Ms Juhl said: "Recent studies on alcohol and fertility suggest that even low levels of alcohol intake are associated with longer waiting times to pregnancy. Our very large study does not show the same association, even for women who consume up to 14 drinks a week. The slightly higher waiting times for women who consumed more than that disappeared after we took account of whether they had given birth before or not. However the results of our study do not provide arguments against the Danish recommended intake of less than 14 drinks a week for women.
"The most consistent finding was a longer waiting time to pregnancy in women who reported no alcohol intake. We do not expect alcohol to improve a woman's fertility, but a moderate intake may correlate with a higher frequency of intercourse, which may explain the longer waiting times in women who reported no intake. Small amounts of alcohol may have a positive impact on the female reproductive system, perhaps by helping women to feel more relaxed.
"However it is important to underline that we only had information on average weekly intake of alcohol before pregnancy. Our results cannot rule out that alcohol intake on specific occasions around the time of ovulation may reduce the chance of becoming pregnant in that menstrual cycle."
* Moderate alcohol consumption and waiting time to pregnancy. Human Reproduction. Vol 16. No 12. pp 2705-2709.
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