As younger women continue to partake in the western coffee culture and demand high-quality coffee beans, the current trends of coffee consumption are expected to continue. The properties of coffee make it a double-edged sword, and the balance between its beneficial and harmful health impacts should be considered. In this study, we examined coffee consumption patterns before pregnancy and their association with the risk of bleeding in early pregnancy among pregnant Korean women. Our study showed that women who were habitual coffee drinkers before pregnancy constituted a larger fraction of those experiencing bleeding in early pregnancy than women who were seldom coffee drinkers. We found that habitual coffee consumption of one or more cup/day before pregnancy was significantly associated with an increased risk of bleeding in early pregnancy, even after adjustment for cigarette smoking and alcohol consumption. However, the type of coffee consumed did not significantly affect the risk of bleeding in early pregnancy.
In the present study, among 3510 pregnant women, the overall prevalence of bleeding in early pregnancy was 18.1%, even though the average maternal age of participants was relatively high, which is consistent with previous results. Bleeding in early pregnancy is associated with an increased risk of poor fetal and maternal outcomes, and perinatal mortality was observed to be more than twice as frequent in women who experienced bleeding in early pregnancy in the meta-analysis than in those who did not . First trimester bleeding could indicate an underlying placental dysfunction, which may be related to later pregnancy complications . Therefore, thorough prevention and management with healthy behavior from preconception to early pregnancy may help to prevent future fetal mortalities and morbidities. However, information on risk factors for bleeding in early pregnancy in the first trimester is insufficient and it is still unclear whether specific effects of caffeine is an independent risk factor for bleeding in early pregnancy [12, 18,19,20,21].
Coffee consumption is one possible risk factor for bleeding in early pregnancy in the first trimester. Even though the market for caffeinated beverages has increased in the past decades, coffee remains the most frequently consumed caffeinated beverage [1, 7]. A standard cup of coffee is generally expected to provide 100 mg of caffeine; however, this varies according to portion size, brewing method, and brand [1,2,3, 6, 7, 22]. Although other beverages have some caffeine content (one cup of tea, 64.0 mg of caffeine; 12 oz. of coke, 46.0 mg of caffeine; one cup of hot chocolate, 16.0 mg of caffeine; and caffeinated soda, 46.0 mg of caffeine), these caffeinated beverages do not significantly affect daily caffeine consumption among Koreans [7, 23]. Nisenblat et al. reported that caffeine intake is not associated with an increased risk of bleeding in early pregnancy, with the possible exception of very high levels of caffeine intake . However, caffeine and its metabolites easily cross the placenta and may be present in considerable quantities in the amniotic fluid and fetal blood [8, 24]. Moreover, the fetus metabolizes caffeine very slowly, and even extremely small amounts of maternal caffeine intake could lead to long-term fetal caffeine exposure [22, 24]. Experimental and human studies have shown that caffeine exposure induces angiotensin II by stimulating the generation of reactive oxygen species, which ultimately inhibit angiogenesis and negatively affect the developing embryo . In addition, caffeine consumption could increase the generation of circulating catecholamines, which could cause uteroplacental vasoconstriction, leading to fetal hypoxia [22, 26, 27]. Moreover, although a threshold for the adverse effects of caffeine on pregnant women was not well established, a few studies showed that high levels of caffeine intake could have adverse effects, such as miscarriage, fetal growth restriction, and long-term behavioral effects in offspring [9, 11, 16, 22, 23, 28, 29].
Recently, some epidemiologic studies have found a significant association between a caffeine intake of 300 mg or more/day and the risk of early pregnancy loss [23, 30, 31]. Consistent with previous reports, in the present study, we found that pregnant women who were heavy coffee drinkers had a significantly higher risk of bleeding in early pregnancy. In a Chinese prospective study, caffeine intake before pregnancy was not found to increase the risk of early pregnancy loss, but caffeine intake of more than 300 mg/day during the first trimester appeared to significantly increase this risk . A UK case-control study showed that caffeine consumption of more than 300 mg/day during pregnancy approximately doubles the risk of miscarriage, and this effect is driven by coffee consumption . Similarly, a study found that the adjusted risk of early pregnancy loss among Danish women who consumed more than 375 mg of caffeine/day was 2.21 .
A meta-analysis found that the risk of pregnancy loss increased by 3% for every increase in coffee consumption of two cups/day . Hence, most women try to reduce their caffeine intake considerably during pregnancy, especially from the time they start preparing for pregnancy to the first trimester [9, 29]. The current guidelines of the World Health Organization recommend a caffeine intake below 300 mg/day, whereas the American College of Obstetricians and Gynecologists recommend a maximum caffeine intake of 200 mg/day [32, 33]. Different recommendations in guidelines can lead to confusion in preparing for pregnancy or during pregnancy. Moreover, knowing the exact caffeine content is difficult, because the amount varies depending on the serving size of the coffee.
The present study has some limitations that should be noted. The main limitation of the present study relates to the inaccurate assessment of the coffee consumption pattern, because the caffeine consumed in a “cup” of coffee varies according to portion size, brewing method, and brand type. Despite the huge popularity of decaffeinated coffee, we could not examine the consumption of decaffeinated coffee, especially in pregnant women. In future studies, objective measurements, combining caffeine exposure biomarkers from blood, urine, and saliva with 24-h dietary recall measurements, should be used to assess precise coffee consumption. Second, recall bias due to the retrospective assessment of caffeine consumption should be considered. However, since we examined coffee consumption before the onset of bleeding in early pregnancy, the impact of recall bias may have reduced. Third, coffee consumption patterns before pregnancy at a single time point may not reflect chronic exposure over the years, because women who prepare to conceive tend to maintain healthy eating habits. To address this concern in future studies, we should measure coffee consumption before pregnancy and evaluate the reliability of the FFQ. Finally, although we controlled for several potential confounders in our analysis, residual confounding by the effects of diet or other lifestyle factors may have been present. Additionally, a causal relationship between coffee consumption and bleeding in early pregnancy could not be assessed due to the observational design of this study.