Women who are trying to become pregnant should avoid alcohol during the second half of their menstrual cycle, research suggests.
No amount of alcohol is safe during pregnancy, with experts recommending women who are trying to conceive also abstain.
This may be particularly important in the two weeks before a woman's period, with scientists from the University of Louisville in Kentucky reporting even a moderate consumption – three to six alcoholic drinks a week – may reduce a woman's chance of conceiving by 44%.
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Although it is unclear why this may occur, alcohol could affect ovulation – stopping an egg from being released.
If ovulation does take place, moderate drinking may also prevent a fertilised egg from being implanted in the womb, which is required for a baby to develop.
About one in seven couples in the UK struggle to conceive.
Women are most fertile around the time of ovulation.
Most women have a period every 28 days, however, anything from 21 to 40 days is considered a normal menstrual cycle – defined as the first day of a woman's period to the day before her second period.
Ovulation generally occurs 10 to 16 days before the start of a woman's period, with the egg surviving for around 24 hours.
A woman becomes pregnant if a man's sperm fertilises the egg. Sperm can survive in the fallopian tubes, which connect the ovaries to the womb, for up to seven days after sex.
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The extent to which alcohol affects this process was unclear, with previous studies being of dubious quality.
To learn more, the Louisville scientists analysed 413 participants of the Mount Sinai Study of Women Office Workers.
The women – aged 19 to 41 – were followed for up to 19 menstrual cycles, between 1990 and 1994.
Alcohol consumption was monitored via daily diaries. The women also provided urine samples on the first and second day of each menstrual cycle to check for pregnancy.
Perhaps unsurprisingly, a heavy intake – defined as consuming more than six alcoholic drinks a week – was linked to a reduced chance of becoming pregnant.
"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," said lead author Dr Kira Taylor.
"During the luteal phase – which is the last two weeks of the menstrual cycle before bleeding would start and when the process of implantation occurs – not only heavy drinking but also moderate drinking was significantly associated with a reduced probability of conception."
Moderate drinking during the luteal phase was linked to a 44% reduced chance of conceiving, compared to the women who never consumed alcohol.
Heavy drinking lowered the chance by 49%, the results – published in the journal Human Reproduction – suggest.
Consuming six or more drinks a week at the time of ovulation cut the odds of conception by 61%.
"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", said Dr Taylor.
"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.
"The effect of moderate drinking during the luteal phase is more pronounced and only about 16 moderate drinkers would conceive".
Bingeing – defined as consuming at least four alcoholic drinks across 24 hours – was also found to affect a woman's pregnancy chances, with every additional day of binge drinking cutting the odds by 19% and 41% during the luteal and ovulation phases, respectively.
The type of alcoholic drink the women consumed was not found to influence their pregnancy chances.
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The scientists have stressed their study was small, with not all the participants trying to conceive.
"Our study only included a few hundred women and while we believe the results strongly suggest heavy and even moderate alcohol intake affects the ability to conceive, the exact percentages and numbers should be viewed as rough estimates," said Dr Taylor.
The women also self-reported their alcohol intake, potentially underestimating their consumption.
In addition, the study was carried out around 30 years ago "and hence the context is no longer current", according to Dr Ying Cheong, from the University of Southampton.
Information was also not collected on the women's male partners' drinking habits.
"Perhaps partners of women who were heavy or binge drinkers were themselves less fertile and that's the association?," said Professor Tim Child, from the University of Oxford.
Dr Gareth Nye, from the University of Chester, agreed with the study's shortcomings, adding: "Ultimately, it is advisable women who are pregnant, or are struggling to get pregnant, limit alcohol intake along with other lifestyle changes such as diet, exercise and caffeine intake.
"This paper does not change that message."
Dr Taylor also pointed out drinking any amount of alcohol does not eliminate a woman's chance of becoming pregnant.
"Alcohol is not birth control", she said. "Even if a woman drinks alcohol heavily, if she has unprotected intercourse, she can become pregnant."