Women who drink five or more cups of coffee a day halve their chance of successful IVF treatment, according to a study.
Researchers, who followed up almost 4,000 In vitro fertilisation and Intracytoplasmic sperm injection (ICSI) patients, described the adverse impact as 'comparable to the detrimental effect of smoking'.
The results showed that drinking five or more cups of coffee a day reduced the clinical pregnancy rate by 50 per cent and the live birth rate by 40 per cent.
Study: Women who drink five or more cups of coffee a day 'halve' their chance of successful IVF treatment, according to researchers who tracked 4,000 patients
Dr Ulrik Schixler Kesmodel, from the Fertility Clinic of Aarhus University Hospital in Denmark, said: 'Although we were not surprised that coffee consumption appears to affect pregnancy rates in IVF, we were surprised at the magnitude of the effect.'
The link between caffeine and fertility has been studied in the past, with conflicting results. Some studies have found an increased incidence of spontaneous abortion in coffee drinkers, but other studies have not.
However, one much-cited study from 2004 showed that time-to-pregnancy was significantly extended in women when coffee or tea intake was more than six cups per day or when the male partner consumed more than 20 alcohol units per week.
The latest Danish study, which was performed in a large public IVF clinic, involved 3,959 women having IVF or ICSI as fertility treatment. Information on coffee consumption was gathered at the beginning of treatment, and at the start of each subsequent cycle.
The statistical analysis controlled for variables including age, smoking habits and alcohol consumption, cause of infertility, female body mass index, and number of embryos retrieved.
The analysis showed that the 'relative risk' of pregnancy was reduced by 50 per cent in those women who reported drinking five or more cups of coffee per day at the start of treatment - and the chance of live birth was reduced by 40 per cent, though this trend was not quite statistically significant.
No effect was observed when the patients reported coffee consumption of less than five cups per day.
'Although we were not surprised that coffee consumption appears to affect pregnancy rates in IVF, we were surprised at the magnitude of the effect'
The researchers compared the adverse effect of five cups of coffee 'to the detrimental effect of smoking'.
Several recent studies and reviews have indicated that tobacco smoking has an adverse effect in IVF on the number of eggs retrieved, and rates of fertilisation, implantation, pregnancy and live birth.
Commenting on the results, Dr Kesmodel proposed that in a study of greater numbers the statistical effect of coffee on IVF delivery results would have most likely been significant, and comparable to the effects seen on pregnancy rate.
He said: 'There is limited evidence about coffee in the literature, so we would not wish to worry IVF patients unnecessarily.
'But it does seem reasonable, based on our results and the evidence we have about coffee consumption during pregnancy, that women should not drink more than five cups of coffee a day when having IVF.
'The fact that we found no harmful effects of coffee at lower levels of intake is well in line with previous studies on time-to-pregnancy and miscarriage, which also suggest that, if coffee does have a clinically relevant effect, it is likely to be upwards from a level of four-to-six cups a day.'
However, Dr Euan Paul, executive director of the British Coffee Association, today said 'no harmful effects were found with lower levels of coffee consumption'.
He said: 'Coffee is one of the most heavily researched commodities in the world. The wealth of scientific data shows that there is no need to cut caffeine or coffee out of the diet completely.
'However for pregnant women or those trying to conceive, an upper limit of 200mg of caffeine per day is perfectly safe. This is the equivalent of two to three cups of coffee but all sources should be considered when reviewing total daily caffeine intake.'
The findings will be presented at the annual meeting of ESHRE, the world's largest event in reproductive science and medicine, in Istanbul.
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