Introduction
Whereas there is a general agreement that alcohol in high doses acts teratogenically, controversies still exist regarding the existence of a ‘safe’ level of alcohol intake during pregnancy.1–3
Thirty years ago, two studies with conflicting results concerning moderate alcohol drinking during pregnancy and risk of spontaneous abortion appeared in the same issue of The Lancet. One study showed no increased risk of first trimester miscarriage with alcohol use, and for second trimester spontaneous abortions the risk was doubled only for women drinking more than one drink per day.4 The other study showed a more than doubling of the risk of spontaneous abortion among pregnant women drinking alcohol more than once a week but less than daily, compared with abstainers.5
Studies on moderate alcohol consumption and fetal death published since then have continued to produce conflicting results. Until recently, it has been a trend that studies conducted in the USA and Canada showed a strong relationship, whereas studies conducted in Europe or Australia failed to demonstrate such an association.1
The aim of this study was to estimate the associations between a low to moderate alcohol intake during pregnancy and fetal death, and to assess the alcohol-related risk of early and later spontaneous abortion, and of stillbirth, respectively.
Methods
The study is based upon data from the Danish National Birth Cohort, in which a total of 100 418 pregnancies were enrolled during the years 1996–2003.
Recruitment to the Danish National Birth Cohort occurred in connection with the first antenatal care visit to the general practitioner, which usually takes place shortly after recognition of the pregnancy. The pregnant woman received written information about the Danish National Birth Cohort at the antenatal visit, and was included as a participant in the cohort when she returned a signed informed consent form to the study secretariat at the Danish Epidemiology Science Centre.
Participants provided information on exposures during the first part of pregnancy by means of a computer-assisted telephone interview scheduled in pregnancy Week 12 or as soon as possible after that date. If the woman had experienced a loss of the pregnancy prior to completion of this interview she was offered an interview that had questions similar to those in the pregnancy interview.
The interviews covered questions on exposures in the first 16 weeks of pregnancy, including information on alcohol consumption, smoking, coffee intake, reproductive history and occupation. More details about this cohort are described elsewhere.6
All women were asked about alcohol consumption before and during pregnancy. They were asked about the average weekly consumption of bottles of beer during pregnancy, followed by a question about the average consumption of bottles of beer before becoming pregnant. Likewise they were asked about average weekly consumption of glasses of wine and glasses of spirit, respectively. The answers were coded as none, less than one bottle/glass per week or the number of bottles/glasses per week.
For this particular study, we used data from all pregnant women who provided interview information, either early in pregnancy (N = 90 165) or after a fetal loss (N = 2552). From these 92 717 women, 874 (0.9%) were excluded from the analyses due to lack of information about gestational age at recruitment or at the end of pregnancy (N = 40), no information on average alcohol consumption (N = 98) or no information on co-variables included in the regression models (N = 736), thus resulting in 91 843 pregnancies eligible for analysis.
The outcome measure of interest was fetal death and its main compartments, i.e. spontaneous abortion and stillbirth. In Denmark, a stillbirth since 2004 has been defined as the birth of a child showing no signs of life and with a gestational age of ≥22 completed weeks. Spontaneous abortion is defined as a non-deliberate fetal death of an intra-uterine pregnancy before 22 completed weeks of gestation.
The pregnancy outcomes were identified from the Civil Registration System, the National Discharge Registry and the participating mothers. By record linkage with the Civil Registration System, we identified live-born offspring from all pregnancies in the cohort. From the National Hospital Registry, which contains information about discharge diagnoses on all citizens treated in a hospital setting, we obtained information about other pregnancy outcomes: ectopic pregnancy, induced abortion, hydatidiform mole and spontaneous abortion and stillbirth (Table 1). In the few cases where we could not identify outcome of pregnancy by these two procedures we used information about outcome of pregnancy obtained from the mother and registered in the Danish National Birth Cohort Study. These concerned almost exclusively spontaneous abortions, not treated in a hospital setting. Gestational age was calculated from the last menstrual period, which was reported in the informed consent form. Permissions from the Danish Data Protection Board and the Scientific Ethics Committees were obtained before initiation of the study.