INTRODUCTION
Alcohol was confirmed as a teratogen in the late 1970s
after observations made in France and the USA in
infants born to alcoholic mothers.1 2 Evidence regarding
the damaging effects of heavy drinking in pregnancy
is now well established. However, there is a lack
of consensus regarding the impact of low intakes on
adverse birth outcomes such as preterm birth and
small for gestational age (SGA), with studies reporting
a wide range and even a protective effect of low
intakes in reviews of the evidence.3–10 This is reflected
in the different country-level policies regarding
alcohol consumption during pregnancy and
highlighted in a recent review by O’Leary et al on
alcohol policies in English-speaking countries.11
Some, such as the USA, recommend abstinence.12
Others advise abstinence but state that small amounts
of alcohol are unlikely to cause harm.13 In the UK, the
Department of Health (DH) recommends that pregnant
women and women trying to conceive should
avoid alcohol altogether and never drink more than
1–2 units once or twice a week.14 The National
Institute for Health and Care Excellence (NICE) additionally
emphasises the advice to avoid drinking
alcohol in the first 3 months of pregnancy as this may
be associated with an increased risk of miscarriage.15
According to the UK Health Survey 2011, 52% of
women of childbearing age who drink exceed the
daily limit of 2–3 units per day and 25% drink more
than twice the recommendations.16 Results from the
most recent UK Infant Feeding Survey (IFS) which
included data from over 15 000 women, showed that
40% drank alcohol during pregnancy but only 3%
drank more than 2 units per week.17
Data suggest that over 40% of pregnancies in the
UK are unplanned.18 19 With such high rates of
unplanned pregnancies and excess drinking, early
pregnancy is likely to be the period of highest
intake for women who are unaware of their pregnancy,
and this could put them and their unborn
baby at risk.
Alcohol crosses the placenta and results in nearly
equal concentrations in the mother and fetus. The
mechanisms whereby alcohol affects fetal growth
and development are complex as these are staged
processes, and the sensitivity of the fetus to alcohol
will likely depend on the timing of the exposure.6
Few studies have taken into account the effect of
timing of alcohol exposure on birth outcomes.
Examination of alcohol consumption before pregnancy
and for all trimesters separately showed conflicting
results as to which period is most sensitive;
some studies found an association between alcohol
intake and SGA and preterm birth at all levels of
exposure, while others suggested no association
even at high levels of intake.20–23
The aims of this paper were to investigate the
relationship between maternal alcohol intake
during pregnancy with both birth weight and gestational
age, and to assess whether these relationships
differed by timing of exposure during pregnancy.
We also aimed to investigate the effect of maternal
drinking prior to pregnancy on birth outcomes.
This was accomplished using data from a prospective
cohort.