Caffeine and Pregnancy

“Pregnant women are advised to drink no more than two cups of coffee a day to cut the risk of giving birth to underweight babies” reports The Times. New research has led the UK Food Standards Agency to reduce its maximum recommended daily caffeine intake during pregnancy to 200 mg, roughly the amount in two cups of instant coffee. This well conducted study does show a link between a higher caffeine intake during pregnancy and lower birth weight. The risk is probably very low and so women keeping to the previous maximum limit of 300mg should not be overly concerned, just reduce caffeine to the new limit.

Guide to Caffeine Content in Food

Caffeine can be found in coffee, tea, colas, and chocolate. Some over-the-counter drugs, including headache and cold tablets, stay-awake medications, and allergy remedies also contain caffeine.

Even the amount of caffeine in coffee and tea can vary widely depending on whether the coffee grounds or tea leaves are brewed or instant, weak or strong. Colas and other drinks vary in caffeine content, too. See the list below for average caffeine amounts in some common foods and beverages

  • 1 mug of instant coffee = 100mg
  • 1 mug of filter coffee = 140mg
  • 1 mug of Tea = 75mg
  • 1 can of cola = up to 40mg
  • 1 can of "energy" drink = up to 80mg
  • 1 x 50g bar of plain chocolate = up to 50mg
  • 1 x 50g bar of milk chocolate = up to 25mg

So if you eat a bar of plain chocolate and drink one mug of filter coffee, or if you drink two mugs of tea and a can of cola you'll have almost reached 200mg.
Source: FSA Website

The research was carried out by members of the CARE study group including researchers from the Universities of Leicester and Leeds. The work was funded by the Food Standards Agency in the UK. The study was published in the peer-reviewed British Medical Journal.

This was a prospective cohort study. The researchers looked at the association between the amount of caffeine pregnant women drink and the weight of their babies at birth. Previous studies have found that caffeine consumption during pregnancy is associated with reduced birth weight, but were unclear on what level of caffeine is associated with this effect.

The researchers enrolled women who were 8-12 weeks pregnant between 2003 and 2006. Women had to be aged 18-45 years old, to be carrying a single baby (i.e. no multiple births), and not to have any medical or psychiatric disorders, HIV or hepatitis B infection. Of the 13,071 eligible women, 2,635 (20%) agreed to participate.

At the beginning of the study, participants were visited by a researcher. Each woman was asked to fill in a standard questionnaire about their caffeine intake for the period starting four weeks before their pregnancy until their enrolment. This questionnaire asked for information about consumption of all possible sources of caffeine (food, drink, and over-the-counter medications), as well as brand names of products used, frequency of use, portion sizes, and methods of preparation.

The researchers identified how much caffeine there was in each item described and estimated each woman’s average daily caffeine.

Women filled in the questionnaire again for the 13th to 28th weeks, and 29th to 49th weeks of pregnancy. Questions asked related also to other factors including nausea, smoking, and alcohol consumption and tests were carried out to prove their accuracy. The researchers also carried out tests to determine how long caffeine stayed in the women’s bodies, by asking them to drink a diet cola containing 63.5mg of caffeine first thing in the morning after an overnight fast, and then testing their saliva one and five hours later.

Once the babies were born, information was obtained about the length of the pregnancy, the baby’s birth weight and gender from computer records. The babies’ birth weights were compared to the expected birth weight range based on standard charts that took into account the mother’s height, weight, ethnicity, and number of previous children and the baby’s gender. Babies whose weights were in the lowest 10% of the expected range were described as having fetal growth restriction (FGR).

The researchers then looked at the risk of FGR and these other outcomes among women with different levels of caffeine intake. They took into account factors that might affect their results, such as maternal characteristics (height, weight, ethnicity, number of previous children, smoking, and alcohol consumption), and duration of the pregnancy. They also looked at what happened if they took into account maternal nausea or how the women metabolised caffeine, or excluded woman with high-risk pregnancies, who had had more than one previous child, or who had very high or low caffeine consumption.

What were the results of the study?

On average, women consumed 159 mg caffeine a day during pregnancy. Most of this caffeine came from tea (62%), with 14% coming from coffee, 12% from cola drinks, 8% from chocolate, 2% from soft drinks, 2% from hot chocolate, 1% from energy drinks, under 1% from alcoholic drinks, and a negligible amount from over the counter medications.

Of the 2,635 women who took part, 343 (13%) had babies with fetal growth restriction (FGR). Higher maternal caffeine intakes during pregnancy were associated with greater risk of FGR in the baby. About 11% of babies of mothers who consumed less than 100 mg caffeine a day had FGR, compared to 13% of those whose mothers consumed 100-199 mg a day, 17% of those who consumed 200-299 mg a day, and 18% of those who consumed 300 mg a day or more.

Babies whose mothers consumed over 200 mg caffeine daily had 40-50% odds of having FGR than those whose mothers consumed less than 100 mg a day. Findings were similar if the researchers looked at caffeine consumption in each trimester separately. Women who consumed over 200 mg of caffeine a day had babies that weighed about 60-70 g less than women who consumed less than 100mg daily.

Women who had reduced their caffeine intake from over 300 mg a day before pregnancy, to less than 50 mg a day by weeks five to 12 of pregnancy had babies with higher birth weights than those women who continued to consume over 300 mg a day.

The researchers conclude, “Caffeine consumption during pregnancy was associated with an increased risk of fetal growth restriction and this association continued throughout pregnancy. Sensible advice would be to reduce caffeine intake before conception and throughout pregnancy.”

This relatively large and well-conducted study provides evidence of an association between caffeine consumption during pregnancy and low birth weight. The fact that caffeine intake from any source was assessed is another strength of this study. There are a few points to consider when interpreting the results:

In light of the findings of this study, women should consider restricting their caffeine intake when they are pregnant. The FSA has suggested that women consume less than 200 mg caffeine a day during pregnancy, this represents about two cups of instant coffee or tea. Women should also remember to count any caffeine containing foods such as chocolate when estimating their intake.

Pregnant women who have stuck to the previous maximum amount of 300 mg should not worry too much as the risks are very small, and simply reduce their intake to the new amount.

Sir Muir Gray adds..."Sounds like sensible advice, based on this evidence".

Links to the Headlines

Pregnant women told to drink no more than two cups of coffee a day »
The Times, November 2008.

Cut caffeine, pregnant women told »
BBC News, November 2008.

Two cups of coffee a day can lead to underweight babies, experts claim »
Daily Mail, November 2008.

Caffeine link to under-weight babies prompts cut to government coffee guidelines »
The Daily Telegraph, November 2008.

New research figures now available