Women who ingest large amounts of caffeine in early pregnancy are significantly raising their risk of miscarriage, according to the results of a Swedish study.

Dr Sven Cnattingius, of the Karolinska Institute in Stockholm, and colleagues at Uppsala University interviewed 562 women who had miscarried at 6-12 weeks of gestation and 953 control subjects matched according to week of gestation. Women with miscarriages had a significantly higher caffeine intake, usually in the form of coffee, than the controls.

Separate analyses adjusted for smoking, age, number of previous pregnancies and miscarriages, alcohol consumption, and presence of nausea, vomiting and fatigue produced further important data. The risk associated with caffeine was not observed in smokers, suggesting that smoking may either conceal the effect of caffeine or increase the rate of caffeine elimination.

However, among non smokers, there were more miscarriages with ingestion of 100-299 mg of caffeine per day (odds ratio 1.3) compared with women who ingested less than 100 mg daily. Among those who consumed 300-499 mg per day, the odds ratio was 1.4, rising to more than twice the risk (odds ratio 2.2) with 500 mg per day or more.

In addition, laboratory analyses found that a high intake of caffeine induced the abortion of otherwise healthy, normal fetuses refuting the argument that a miscarriage is always the body's rejection of an abnormal fetus (N Engl J Med, 2000; 343: 1839-45).

Overrun for Updates

In a related US study, maternal caffeine consumption in the first trimester was associated with an increased risk of miscarriage in women who experience nausea.

Researchers found that women who consumed 20-99 mg of caffeine daily during the first trimester had a miscarriage risk ratio of 1.5 compared with women who consumed less than 20 mg. This rose to 2.0 for women who consumed 100-299 mg per day and to 2.5 for those ingesting 300 mg or more daily.

But, among women who experienced nausea during pregnancy and consumed at least 300 mg of caffeine daily, the risk of miscarriage was more than doubled (risk ratio 5.4).

It is possible, say the researchers, that caffeine consumption may result in a decrease in oestrogen levels, which could allay the discomfort of pregnancy, decreasing the severity and duration of nausea and, subsequently, increasing the risk of spontaneous abortion (Epidemiology, 2001; 12: 38-42).