Tuesday November 4 2008
“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. The newspaper says that 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. Women should aim to restrict their caffeine intake in line with the new recommendations during pregnancy. 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.
Caffeine is also present in tea, chocolate, soft drinks, energy drinks, and some medicines such as cold and flu remedies.
Where did the story come from?
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.
What kind of scientific study was this?
This was a prospective cohort study. In it, 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 at two UK hospital maternity units, between 2003 and 2006. To be eligible, 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 at the hospital, at home, or at their GP surgery 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 of pregnancy, and the 29th to 49th weeks of pregnancy. The questionnaires also asked about factors that might affect caffeine consumption and birth weight, including nausea, smoking, and alcohol consumption. To test the accuracy of the women’s reports of their smoking, a saliva test for the chemical cotinine (a chemical formed when nicotine is broken down) was carried out at the start of the study. 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 participants’ babies were born, the researchers obtained information about the length of the pregnancy, and 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 also collected information about outcomes such as high blood pressure during pregnancy (with or without protein in the urine), late miscarriage (between 12 and 24 weeks), preterm delivery (before 37 weeks), and stillbirth (birth at 24 weeks or later with no sign of life).
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.
After possible confounders were adjusted for, babies whose mothers consumed 100-199 mg daily were at a 20% increased risk (odds) of having FGR compared to babies of mothers who consumed less, but this increase was not statistically significant. 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.
What interpretations did the researchers draw from these results?
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.”
What does the NHS Knowledge Service make of this study?
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:
- Only 20% of the women invited to take part did so, which is a relatively low rate. However, the researchers did not think that this 20% of women would differ from the general population, as the participants did not differ greatly from the overall population in the two maternity units.
- Women had to remember and report their consumption of foods, drinks, and medications containing caffeine and errors could have been introduced at this point. However, the periods they were asked about were relatively recent and not too long; therefore recall should have been relatively good. The fact that that the researchers used a standard questionnaire that had previously been tested increases the likelihood of getting reliable results. Also, the fact that caffeine consumption was assessed before the birth of the baby means that the womens’ recollection would not have been affected by this knowledge.
- It is possible for studies of this type to be affected by factors that are unbalanced between the groups that are compared. For example, if caffeine did not affect birth weight, but women who consumed high levels of caffeine also drank more alcohol, then (as alcohol affects birth weight) an association between caffeine and birth weight would be found if alcohol use was not adjusted for (taken into account). The authors adjusted for factors that they knew might affect results, such as maternal smoking, alcohol use, and other maternal characteristics. These adjustments increase the likelihood that the association seen between caffeine and birth weight is real, but there may still be other factors which are having an effect that were not measured.
- The authors point out that being in the lowest 10% of birth weights does not indicate that there was necessarily anything medically wrong with the babies.
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.