Wednesday June 30 2010
We cannot tell if drinking actually caused infertility
“Pregnant women who drink alcohol may reduce sperm count of sons,” according to The Guardian. It reported that research has found that pregnant women who drank more than 4.5 alcoholic drinks a week were more likely to have sons who had a lower sperm count than women who drank little alcohol.
This Danish research has analysed the pregnancy drinking habits of 347 women during pregnancy and the quality of their adult sons’ semen. Although the study found a relationship between higher alcohol consumption and lower sperm concentration, semen volume and total sperm count, the trend and its implications are not completely clear. There are numerous limitations to the research, such as the small number of participants and the adaptation of a study design originally devised to examine smoking. Crucially though, male fertility was not directly assessed, meaning it is incorrect to assume that the men involved would have difficulties if they try to father children.
Overall, there are no definite conclusions to draw from this limited research. However, regardless of the study’s limitations, pregnant women are strongly advised to limit or avoid alcohol during pregnancy due to the numerous established harmful effects of excess alcohol during pregnancy.
Where did the story come from?
The study was carried out by researchers from Aarhus University Hospital, Denmark, and funded by Danish Medical Research Council. The study was published in the peer-reviewed medical journal, Human Reproduction.
News reports have reflected the findings of this research, but have generally not taken into account several of the study’s important limitations. This means that no firm conclusions can be drawn on this issue. The Daily Mail’s opening paragraph, which says that ‘pregnant women who drink alcohol could be jeopardising their chances of becoming grandmothers,’ is not substantiated by this research or supported by the researchers.
What kind of research was this?
This was a cohort study that aimed to investigate the effects of exposing a male foetus to alcohol. Specifically, it looked at what effect maternal alcohol consumption during pregnancy has upon sperm quality and levels of reproductive hormones once the child has reached adulthood.
A cohort study is the best design for examining the relationship between a cause (maternal alcohol) and potential effect (reduced fertility in the son). However, to ensure the accuracy of its results a cohort study must take into account all possible confounders that could affect the relationship being studies. A limitation of this particular study is that it was not set up to examine the link between maternal alcohol consumption during pregnancy and sperm quality in the son. The original aim and design was an examination of the effect of smoking in pregnancy on sperm quality.
What did the research involve?
This research used participants of a Danish cohort study (the Healthy Habits for Two study), which recruited 11,980 pregnant women between 1984 and 1987. At 36 weeks of pregnancy the women completed a questionnaire on their lifestyle habits including drinking of beer, wine and spirits. Responses were categorised as drinks per week (for each drink type): never, 1, 1-4, 5-9, 10-14, 15-19, 20 or more.
After summing the total of each type of drink, they put each woman into a category of: less than one drink a week, one to 1.5 drinks a week, two to four drinks, or 4.5 or more drinks a week. One standard drink in Denmark reportedly corresponded to 12g of pure alcohol. In the UK, one standard drink (unit) contains 8g of pure alcohol.
In 2004, a total of 5,109 sons were identified through the Danish Civil Registration System. Between 2005 and 2006, the researchers collected semen samples from 347 men (48.5% of the 716 invited to participate) and took blood samples (both performed with due laboratory protocols). They analysed semen for sperm concentration and motility, and looked at hormone concentrations in the sample.
The men also provided questionnaires containing health and lifestyle questions, including about their own alcohol consumption. When calculating the associations between maternal alcohol and semen quality the researchers adjusted for maternal smoking, and in the sperm donor’s, smoking, alcohol, history of reproduction infections/disease, and days of sexual abstinence prior to providing the sample.
What were the basic results?
Of the mothers of the 347 men who participated in the study, 110 drank less than one drink a week during pregnancy, 127 had one to 1.5 drinks a week, 72 women had two to four drinks a week, and 38 drank 4.5 or more drinks a week.
There was a trend for decreasing sperm concentration with increasing alcohol exposure while in the uterus. The researchers calculated that sons of mothers who were in the highest alcohol category during pregnancy (more than 4.5 drinks a week) had a 32% lower sperm concentration than those whose mothers were in the lowest category (less than one drink a week).
Maternal alcohol consumption showed no clear relationship with either semen volume or total sperm count (the 1-1.5 drinks per week group was associated with the highest volume and sperm count). There was no observed association between maternal alcohol consumption and hormone levels, sperm motility or sperm morphology. They also found that higher pregnancy alcohol consumption was independently associated with the mum being of lower BMI, being of older age and being a smoker, and with the son being of lower birth weight.
How did the researchers interpret the results?
The researchers conclude that their results indicate that prenatal exposure to alcohol may have an adverse effect on sperm production, and if this were a causal relationship it could explain some of the reported differences in semen quality between populations and across generations.
This research has found some association between semen quality in sons and their mothers’ alcohol consumption during pregnancy. However, there are several important limitations to this research:
- As the researchers say, ‘the participants were selected according to levels of maternal smoking during pregnancy’. Carrying out a post hoc analysis that was not the primary aim of the study increases the risk of chance findings. This may be particularly problematic in this instance as the initial research had a preference for selecting women who smoked and therefore may not have been a typical representative sample of pregnant women.
- Although the cohort of pregnant women was very large (11,980), there were only a total of 347 sets of mothers and sons across the four categories of alcohol consumption analysed. With this small number there is a high possibility of chance findings, particularly with the association found for drinking more than 4.5 drinks a week as there were only 38 women and their sons in this category. The findings based on the analyses of these small numbers may be by chance.
- Additionally, only half of the men invited to participate chose to do so. There may be important differences between the population studied and those who chose not to participate.
- An association was found between higher and drink consumption and decreasing sperm concentration, semen volume and sperm count. However, these relationships were not completely clear, with the highest values being in sons of mothers who drank 1-1.5 drinks a week rather than in those who drank more or less than this. There was also no relationship with hormone levels, sperm motility or sperm morphology. Therefore the actual implications of these findings are not clear.
- It is not known whether any of the differences in sperm quality seen across the groups would cause any actual fertility problems for the man.
- Alcohol consumption was assessed at the end of pregnancy. It is unclear whether the answer reflected the whole of the pregnancy, or just at the time of assessment. Also with any assessment like this, the number of drinks and size and strength of a drink will mean different things to different people.
- There is a possibility that other confounders have not been adjusted for or not fully adjusted. For example, the reporting of alcohol consumption by the men themselves was adjusted for but there may have been insufficient data to do this reliably.
Regardless of the limitations of this research and uncertainty over its findings, there are numerous other established harmful effects of consuming alcohol during pregnancy. NICE recommendations on alcohol consumption (based on one unit being 8g of pure alcohol instead of the 12g used in this study) during pregnancy advise that:
- Pregnant women and women planning a pregnancy should avoid drinking alcohol in the first three months of pregnancy because it may be associated with an increased risk of miscarriage.
- If women choose to drink alcohol during pregnancy they should be advised to drink no more than 1 to 2 UK units once or twice a week (1 unit equals half a pint of ordinary strength lager or beer, or one shot [25ml] of spirits. One small [125ml] glass of wine is equal to 1.5 UK units). Although there is uncertainty regarding a safe level of alcohol consumption in pregnancy, at this low level there is no evidence of harm to the unborn baby.
- Women should be informed that getting drunk or binge drinking during pregnancy (defined as more than five standard drinks or 7.5 UK units on a single occasion) may be harmful to the unborn baby.