1 Danish Epidemiology Science Centre at the Department of Epidemiology Research, Statens Serum Institut, 5 Artillerivej,DK-2300 Copenhagen S, 2 Danish Epidemiology Science Centre at the Department of Epidemiology and Social Medicine, 6 Vennelyst Boulevard, DK-8000 Aarhus C, 3 Department of Social Medicine, Institute of Public Health, University of Copenhagen, 3 Blegdamsvej, DK-2200 Copenhagen N and 4 Centre for Alcohol Research, National Institute of Public Health, 25 Svanemøllevej, DK-2100 Copenhagen Ø, Denmark
5 To whom correspondence should be addressed. e-mail: mju{at}ssi.dk
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Abstract |
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Key words: alcohol/beverage preference/birth cohort/fecundity/time to pregnancy
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Introduction |
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For public health matters it is desirable to know whether these findings are related to alcohol, the type of drink or to characteristics of the drinker. To our knowledge, no infertility studies have had the power or data to examine the effect of specific types of alcohol intake, or the data to adjust for confounding related to lifestyle.
It has been suggested that moderate wine drinkers are at lower risk of lung cancer (Prescott et al., 1999), digestive tract cancer (Grønbæk et al., 1998
), stroke (Truelsen et al., 1998
) and overall mortality (Grønbæk et al., 1995
) compared with both non-drinkers and moderate drinkers of beer and spirits. It is under discussion whether these associations are related to specific components in wine, such as antioxidants or similar substances (Tjønneland et al., 1999
).
The aim of this study was to examine the relation between specific types of alcohol consumption and waiting time to pregnancy.
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Materials and methods |
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We estimate that 35% of all pregnant women in Denmark participated in the cohort study, which corresponds to
60% of those invited.
Measurement of exposure and potential confounders
All participants were asked the question: How many ordinary beers/glasses of wine/glasses of spirits did you drink per week before you became pregnant? One bottle of Danish beer contains 11.6 g of alcohol, and 12 g is an approximate average for one glass of wine (one-sixth of a bottle) or serving (4 cl) of spirits. If the woman reported to drink one type of beverage, but not on a weekly basis, this was coded as half a unit per week.
We did not have data on smoking before pregnancy, but if the participants reported to have smoked at any time in the first trimester we considered them to be smokers during waiting time to pregnancy. The rest were categorized as non-smokers.
BMI was calculated on the basis of the womens reporting of their height and weight before pregnancy.
In the interview, use of oral contraceptives was reported only for the period up to 4 months before pregnancy.
The social variable used was generated from data on occupational status as a hierarchical type of variable with six categories: leaders and highly educated, middle range educated, self-employed and office or welfare workers, skilled and unskilled workers, students, and unemployed.
Measurement of outcome
The outcome measure was waiting time to pregnancy based upon the following question: How long time did you try to become pregnant before you succeeded? followed by fixed answering categories: 02 months, 35 months, 612 months and >12 months. The women were also asked whether the pregnancy was planned, partly planned or not planned. If the pregnancy was not planned they were not asked about waiting time to pregnancy.
Statistical analysis
The association between wine, beer and spirit consumption and waiting time to pregnancy was estimated for a waiting time >12 months by means of logistic regression. Since estimates were similar at different cut-off levels, we combined the results using a discrete time survival method, according to the method of Kalbfleisch and Prentice (1980). We used a logistic model to estimate the conditional probability of not getting pregnant within each waiting time interval given that the subject was not pregnant in the previous intervals. The intervals were 02 months, 35 months, 612 months and >12 months. All effect measures are given in subfecundity odds ratios, where a high odds ratio indicates a longer waiting time to pregnancy. Within each beverage type group we used the abstaining group as reference. In the model we included wine, beer and spirits consumption, potential confounders such as age, parity, smoking, thinness (BMI <18.5 kg/m2), obesity (BMI >30 kg/m2), pelvic inflammatory diseases, occupational status and a categorical variable to indicate the waiting time interval.
In addition, the discrete time survival analysis was replicated including the partly planned pregnancies. Analyses were also made with the use of preference groups (wine, beer, spirits, mixed and abstainers) and drinking pattern (only wine, wine and beer, wine and spirits, beer and spirits, and wine, beer and spirits) as explanatory variables. Stratified analysis was made on possible confounders. Since our data on the use of oral contraceptives were restricted to the last 4 months before pregnancy, this variable could not be used as adjustment variable. We examined a possible correlation between use of oral contraceptives and drinking habits in women with the shortest waiting time interval (02 months).
Analyses were carried out using SPSS 10.0 software.
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Results |
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When we grouped the women according to their drinking patterns, we found that women who drank only beer or only spirits waited longer to become pregnant than all other combinations (only wine, wine and beer, wine and spirits, beer and spirits, and wine, beer and spirits) (data not shown). Women who drank all three types of alcohol had the shortest waiting times to pregnancy.
Women who had suffered from any kind of abdominal or pelvic inflammatory diseases (others than the already excluded, i.e. endometriosis, ovarian cancer and cervical cancer) waited longer to become pregnant than those who had not. No significant interaction was seen between type of alcoholic beverage and pelvic diseases (data not shown).
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Discussion |
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The estimates for spirits consumption should be viewed with caution, since these groups included few pregnancies, especially the highest group with an intake of more than seven drinks per week (27 women).
Our study excluded infertile couples; therefore, proper fecundity effect measures cannot be estimated. Consequently, the rationale behind the estimates we present is that alcohol exposure may prolong waiting time rather than cause sterility. We expected alcohol to have such an effect, if it has any effect at all.
We have not been able to find published studies in fecundity research that distinguish between different types of beverages. Nevertheless, our results support earlier studies suggesting some beneficial health effects of wine drinking (Grønbæk et al., 1995; Grønbæk et al., 1998
; Truelsen et al., 1998
; Prescott et al, 1999
). The wine drinking effect could be explained by residual confounding or uncontrolled confounding. If wine drinkers differ from othersif they, for example have fewer infections that cause sterility, have more sexual contacts, have more appropriate timing of intercourse, or have partners with better sperm qualitythey would have shorter waiting times.
It has been shown that different drinking habits are strongly related to dietary habits. In a Danish study wine drinking was associated with an intake of healthier food (Tjønneland et al., 1999). Although we know very little about the association between dietary components and fecundity, these findings should be taken into account. A recent study showed that wine drinkers are at lower risk of becoming heavy or excessive drinkers than beer drinkers (Jensen et al., 2002
), which supports the common opinion that beverage preference is associated with personality and lifestyle habits that can be difficult to adjust for fully.
Data on smoking before pregnancy were not available. We assumed that women who smoke during pregnancy also smoked before pregnancy, and that they were more likely to have been regular smokers and not just occasional smokers. The association with wine drinking may be underestimated if smoking is associated with larger waiting times, since fewer smokers were found among wine drinkers.
We found, as expected, that women who had had pelvic inflammatory diseases or abdominal diseases waited longer to become pregnant. However, the risk of subfecundity according to these diseases was not related to beverage preference. Although this finding argues against confounding by infection, it does not rule out residual confounding.
Use of oral contraceptives could be a confounding factor, but a correlation was only seen between non-users and women who reported no spirits consumption. Since use of oral contraceptives was reported only for a period up to 4 months before conception, we cannot adjust for this. Any confounding effect is expected to be small for waiting times of >6 months.
Socio-economic status is related to drinking habits, but does not seem to be associated with fecundity. A previous Danish study on 700 women showed no significant associations between subfecundity and family net income or employment status (Rachootin and Olsen, 1982). In our study, socio-economic status was measured by occupational status, and including this variable did not alter the estimates.
Self-reported data on alcohol intake are in general assumed to be under-reported. If there is a modest beneficial effect on fecundity associated with wine drinking, and under-reporting takes place, the effect may be underestimated.
A beneficial effect of wine drinking raises the question of whether such an association can be explained by a differential reporting pattern for the different types of alcoholic beverages. We have no indication to show that a possible under-reporting correlates with beverage preference. A previous report found no such association (Grønbæk and Heitmann, 1996).
The association between wine drinking and waiting time to pregnancy in this study was not very strong, and there was no clear doseresponse pattern. We encourage others to check available data to see whether they find a similar beneficial effect of wine drinking.
In conclusion, our findings suggest that drinking wine may be associated with a modestly decreased risk of subfecundity. Subfecundity did not appear to be related to beer or spirits consumption.
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Acknowledgements |
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References |
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Submitted on February 3, 2003; resubmitted on May 1, 2003; accepted on May 28, 2003.