Variability of self reported measures of alcohol consumption: implications for the association between drinking in pregnancy and birth weight.

STUDY OBJECTIVE

To describe the intra-subject variability of self reported maternal alcohol consumption using different ways of collecting information and to analyse the implications of this variability for research into the effect of low to moderate maternal alcohol consumption on birth weight.

DESIGN

This was a longitudinal study. Self reported maternal alcohol consumption before, during, and after pregnancy was assessed on four occasions over two years. The data were collected by two self administered questionnaires and during two personal interviews (one by phone and another face to face).

SETTINGS

The Obstetrics Department, Odense University Hospital, Odense, Fünen, Denmark.

PARTICIPANTS

A total of 2880 pregnant women were recruited consecutively from the hospital catchment area. Altogether 328 pregnant women and their babies were selected. All women who reported an average alcohol consumption of five drinks or more per week were recruited to the study (164 women) and a 1:1 control group was selected from the remaining women based upon two matching criteria: expected date of delivery and the women's year of birth. Some 279 women (85%) completed the study.

MEASUREMENTS AND MAIN RESULTS

Self reported alcohol consumption (number of drinks per week) and birth weight (g) were the main outcomes. Women's self reported alcohol consumption varied over time and according to the data collection method. When different methods of data collection were used to assess alcohol intake in similar periods of time, significant differences in reporting were found despite the relatively high correlations between the measurements. Although a consistent reduction in birth weight with increasing consumption of alcohol was found, there were differences in the shape and strength of this association when comparing the six available alcohol measurements.

CONCLUSIONS

The type of questions used, the way the data were collected, the period of time referred to, and the time the questions were asked, should be taken into consideration when describing the drinking pattern of pregnant women. Furthermore, birth weight results from studies that have used different alcohol measures should be interpreted or compared with caution because of possible large differences resulting from the differing methods of assessing fetal exposure to alcohol.