Caffeine intake in early pregnancy
A critical aspect of caffeine exposure includes the importance of measuring exposure to caffeine during the relevant time window and the need to capture changing intake patterns throughout pregnancy. Caffeine consumption tends to decrease during the early weeks of pregnancy, coinciding with increasing pregnancy symptoms and aversions1.
Pregnancy symptoms, including aversions to tastes and smells, nausea and vomiting are common in healthy pregnancies that result in live births, and occur less frequently among women whose pregnancies end in miscarriages. This relationship is attributed to a stronger pregnancy signal linked to higher concentrations of pregnancy hormones in viable pregnancies4.
Caffeine consumption has been shown to decrease with increasing pregnancy symptoms during the early weeks of pregnancy1. For example, research from a group of pregnant women suggested that the mean onset of nausea, vomiting and appetite loss occurred between 5 and 6 weeks from the last menstrual period, and was accompanied by a 59% decrease in caffeine intake from coffee between weeks 4 and 62.
It may be that women experiencing healthy pregnancies are more likely to reduce their caffeine intake in response to pregnancy symptoms than women who will have a miscarriage. As a result, reduced caffeine consumption may be a consequence of pregnancy viability as opposed to increased consumption causing any reproductive complication (“reverse causation”).
One study describes patterns of dietary caffeine consumption before and after pregnancy recognition in a cohort of 8,347 American women who had recently given birth3. Maternal self-reported consumption of beverages (caffeinated coffee, tea, and soda) and chocolate the year before pregnancy was used to estimate caffeine intake. About 97% of mothers reported caffeine consumption (average intake of 129.9 mg/day the year before pregnancy); soda was the primary source of caffeine. The proportion of mothers reporting dietary caffeine intake of more than 300mg/day was significantly higher among those who smoked cigarettes or drank alcohol. Most mothers stopped or decreased their caffeinated beverage consumption during pregnancy. A high level of caffeine intake was associated with risk factors for adverse reproductive outcomes. The authors recommend further studies which may improve the maternal caffeine exposure assessment by acquiring additional information regarding the timing and amount of change in caffeine consumption after pregnancy recognition.
Data from the UK Caffeine and Reproductive Health (CARE) study were used to explore the relationship between maternal caffeine intake and nausea, vomiting and fetal growth restriction in pregnancy32. No suggestion of a relationship could be found. The strength of this study is the thorough assessment of caffeine exposure, however the extremely low response rate (20%) is a concern and a selection bias cannot be excluded.
Coffee and Gestational Diabetes Mellitus
Gestational diabetes (or gestational diabetes mellitus, GDM) is a condition in which women without previously diagnosed diabetes exhibit high blood glucose levels during pregnancy (especially during their third trimester).
A population-based cohort of 71,239 women taking part in the Danish National Birth Cohort examined the relation between first trimester coffee and tea consumption and gestational diabetes mellitus (GDM) risk33. Coffee or tea intake was reported in 81.2% of the women (n = 57 882). 1.3% (n = 912) of pregnancies were complicated by GDM, and among non-consumers, 1.5% of pregnancies were complicated by GDM. After adjustment for age, socio-occupational status, parity, pre-pregnancy body mass index, smoking, and cola intake, there was suggestion of a protective, but non-significant association with increasing coffee. Results were similar by smoking status, except a non-significant 1.45-fold increased risk with ≥8 coffee cups/day for non-smokers. There was a non-significant reduced GDM risk with increasing total caffeine. These results suggest that moderate first trimester coffee and tea intake is not associated with GDM increased risk; it may even have a protective effect33.
Caffeine and miscarriage
The epidemiological studies evaluating the risk of miscarriage, also known as spontaneous abortion, from caffeine exposure have been inconsistent1,13. However, many of these studies failed to evaluate the pregnancy signal.
A study of maternal caffeine consumption, nausea and spontaneous abortion suggests that an increased risk of miscarriage was only observed for caffeine consumed after nausea onset, but not for caffeine consumed before nausea onset, or among those without nausea20.
Other persistent problems with the validity of studies of caffeine and miscarriage include confounding by smoking and potential recall bias, as suggested in a 2008 cohort study21. This study is characterised by incomplete control for confounding by the daily number of cigarettes smoked or the duration of nausea and vomiting (only yes/no answers). In addition, this study was only stratified for two levels of caffeine intake, lower or higher than 200mg daily and the latter group includes very high levels of caffeine intake.
In 2010, a Chinese case-control study22 and a small US prospective cohort study23 did not find any association between caffeine consumption and the risk of miscarriage.
In contrast, a UK study reported that greater caffeine intake is associated with an increase in late miscarriage and stillbirth. However, they identified small numbers of late miscarriages and stillbirths, hence limiting the power to detect small associations and leading to considerable uncertainty in the size of the association24.
The 2010 Committee Opinion of the American College of Obstetricians and Gynecologists stated that “Moderate caffeine consumption (less than 200mg per day) does not appear to be a major contributing factor in miscarriage; … a final conclusion cannot be made as to whether there is a correlation between high caffeine intake and miscarriage”34.
A dose response meta-analysis of 130,456 individuals suggested that higher maternal caffeine intake was associated with a higher risk of pregnancy loss, although adjustment for smoking and pregnancy symptoms may have been incomplete. The authors also advised that adherence to guidelines to avoid high caffeine intake during pregnancy appears prudent25.
However, a prospective cohort study of 5,921 women suggested that pre-conceptional caffeine consumption was not associated with an increased risk of spontaneous abortion. The authors did note that consumption during early pregnancy was associated with a small increased risk, although the relation was not linear26.
A further systematic review and meta-analysis suggested that coffee and caffeine consumption was associated with an increased risk of spontaneous abortion at intakes of 300mg and 600mg a day27. The authors expressed their support for advice to limit caffeine intakes during pregnancy in line with the EFSA recommendations of a maximum of 200mg of caffeine per day5.
Data from the Nurses’ Health Study of 15,590 pregnancies concluded that pre- pregnancy coffee consumption at levels above 4 cups of coffee per day was associated with an increased risk of spontaneous abortion, particularly at weeks 8-1928.
Caffeine and pre-term birth
Large studies considering total caffeine intake have consistently reported no increased risk of delivery before 37 weeks of gestation1. This was confirmed by a meta-analysis, including 15 cohort studies and 7 case-control studies, where no important association between caffeine intake during pregnancy and the risk of preterm birth was observed11.
In addition, the 2010 Committee Opinion of the American College of Obstetricians and Gynecologists stated that “Moderate caffeine consumption (less than 200mg per day) does not appear to be a major contributing factor in preterm birth”34.
A Polish study published in 2012 had the objective of estimating maternal caffeine intake during pregnancy and its influence on pregnancy duration, birthweight and the Apgar score of the newborn12. It concluded that caffeine intake of no more than 300mg per day during pregnancy does not affect pregnancy duration and the condition of the newborn. Black tea was the major source of caffeine, whereas 26% of women gave up coffee during pregnancy. An underestimation of maternal caffeine intake during pregnancy could not be excluded, since the questionnaire was carried out on the last day before delivery12.