Background When examining the association between prenatal alcohol exposure and fetal effects, the timing and intensity of exposure have been ignored in epidemiological studies. The effect of using dose, pattern and timing of consumption (“composite” method) was investigated in this study, to examine the association between prenatal alcohol exposure and fetal effects.

Methods The composite method resulted in six categories of exposure (abstinent, low, moderate, binge <weekly, binge 1–2×/week and heavy). The odds of language delay and child behaviour problems were calculated for the composite method and then compared with an analysis using averaged estimates of <1 and 1+ drinks per day and with stratification by quantity ignoring dose per occasion. Data used for the analyses were from a 10% random sample of non-Indigenous women delivering a live infant in Western Australia (1995–1997). Participants from the 1995-1996 cohort were invited to participate in an 8-year longitudinal survey (78% response rate n=2224; 85% were followed-up at 2 years, 73% at 5 years and 61% at 8 years).

Results The effect of moderate and binge levels of exposure was only evident with the composite method; anxiety/depression following first-trimester moderate exposure (OR 2.24, 95% CI 1.16 to 4.34), and following late pregnancy moderate (aggressive behaviour OR 1.93, 95% CI 0.91 to 4.09) and binge (language delay OR 3.00, 95% CI 0.90 to 9.93) exposures. Results for heavy levels of exposure were similar with each method. The estimates for late pregnancy were imprecise due to small numbers.

Conclusion The composite method of classification more closely reflects real-life drinking patterns and better discriminates maternal drinking than the other methods, particularly low, moderate and binge levels.