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Diet and lifestyle

Pregnant women miscarry more often if they smoke or drink too much coffee

Pregnant women are inundated by advice, much of which is well founded. But some of it is based on fads and supposition. A Danish-led research project has now attempted to quantify the dangers of smoking and of drinking coffee during pregnancy. Both smoking and drinking too much coffee separately increase the risk of miscarriage or stillbirth.

Women may have more time to visit local cafés with friends during pregnancy, but perhaps these women should drink smoothies or water instead of the extra caffè lattes. A survey of pregnant women’s consumption of coffee and cigarettes shows that they should definitely avoid smoking cigarettes and should keep their coffee consumption low to minimize the risk of miscarriage or stillbirth.

“The risk of fetal death (miscarriage or stillbirth) is 23% higher if you smoke more than 10 cigarettes a day during pregnancy compared with women who neither smoke nor drink coffee. Similarly, the risk is 17% higher if you drink more than three cups of coffee a day. If you both smoke heavily and drink a lot of coffee, the increased risk climbs to 85%,” explains a main author, Bodil Hammer Bech, Associate Professor, Department of Public Health, Aarhus University.

Smokers metabolize caffeine more rapidly

The study examined data from more than 90,000 women in the Danish National Birth Cohort. The researchers hoped to shed new light on any links between smoking and coffee consumption because a possible connection between them influencing the risk of miscarriage or stillbirth had previously been reported.

“We know that pregnant women metabolize caffeine more slowly and that caffeine can pass through the placenta and thereby affect the fetus. Previous research among smokers also showed that they metabolize caffeine more rapidly. We were therefore eager to determine what the effects of combining smoking and drinking coffee were.”

If the substances in smokers’ blood increase the metabolism of caffeine and if caffeine increases the risk of miscarriage, the researchers would expect that smokers would have a reduced association between coffee and miscarriage. The researchers only found this effect for women who smoked heavily and who drank less than three cups of coffee per day.

“We found a slight compensating effect among women who smoked more than 10 cigarettes per day, but only if they drank less than three cups of coffee per day, but the results were not statistically significant. In contrast, if they drank more than three cups per day and smoked more than 10 cigarettes, the risk of miscarriage almost doubled compared with women who neither smoked nor drank coffee.”

Pregnancy: a vulnerable time

Although the researchers are certain that both cigarettes and too much coffee separately increase the risk of miscarriage or stillbirth among pregnant women, they are more cautious about the combined effect of coffee and cigarettes. More detailed surveys are lacking that could determine why, for example, the risk of miscarriage among women who do not drink coffee but smoke more than 10 cigarettes per day only increases by 5% compared with women who neither smoke or drink coffee.

“These results are not statistically significant and are therefore only an indication, and in any case there is no clear preventive effect. So overall, I think it is better to stick with the precautionary principle that pregnant women should definitely avoid smoking cigarettes and only drink a moderate amount of coffee.”

In the meantime, the researchers are continuing to investigate the factors that influence health and disease among mothers and children during and after pregnancy. They are focusing on prescription drugs, environmental toxins and coffee and cigarettes in this case. Pregnancy is a vulnerable time, and many factors can result in disease or deformity among children.

“Several factors can influence the outcome of pregnancy, or they can emerge later in life. Our research aims to discover the most important factors, because we hope that more precise information about how behaviour before and during pregnancy can affect the development and health of children will enable us to provide even more qualified advice to pregnant women,” concludes Bodil Hammer Bech.

Potential combined effects of maternal smoking and coffee intake on foetal death within the Danish National Birth Cohort” has been published in the European Journal of Public Health. In 2016, the Novo Nordisk Foundation awarded Bodil Hammer Bech a grant for the project Maternal Smoking during Pregnancy and Offspring Utilisation of Healthcare Services.

Bodil Hammer Bech
Associate professor
It is difficult to assess if the association between caffeine intake and outcomes are because of the caffeine in itself or if because the pregnancies were in some way less healthy from the beginning. Another potential risk factor for foetal death is smoking. If a causal association actually exists between smoking and foetal death, the association is not strong. The objective of our studies is to examine a potential interaction on the additive scale between smoking and caffeine intake derived from coffee consumption on risk of foetal death, including SAB and stillbirth.