Caffeine is a stimulant existing naturally in certain plants. It can also be manufactured synthetically and added to food products. Excessive intake of caffeine may be associated with a number of side-effects including increased heart rate, restlessness and anxiety, tremors and difficulty sleeping (Medline Plus website). Caffeine is metabolized by the liver fairly rapidly before being removed from the body by the kidneys.
Caffeine elimination is reduced in pregnant women
A study by Knutti et al (1981) found that the half life for the removal of caffeine from the body during pregnancy was more than doubled from an average of 3.4 hours in non-pregnant females to 8.3 hours in the pregnant subjects. After delivery, the value returned to normal, usually within a month. This study suggests that pregnant women are likely to experience a significantly larger build up of caffeine in their body during pregnancy if they do not moderate their intake. Pregnant women are likely to suffer ill-effects caused by caffeine at much lower doses than non-pregnant women.
Possible risks of moderate caffeine intake
Due to the reduced capacity for the metabolism of caffeine during pregnancy, and the fact caffeine is known to cross the placenta, there have been concerns about caffeine intake during pregnancy since the 1980s. Many epidemiological studies have been carried out investigating possible links between caffeine intake and spontaneous abortion or low birth weight.
In 1998, Fernandes et al combined many of these studies in a meta-analysis in order to establish any possible link. Of the 32 available studies on the effect of caffeine in pregnancy, 12 were deemed to contribute valid data, contributing a total of almost 43 000 pregnancies to the analysis. The researchers found that drinking caffeine appeared to cause a small but significant increase in the risk of miscarriage or low birth weight.
What constitutes a moderate caffeine intake?
In the study by Fernandes, a moderate caffeine intake was taken to be in excess of 150mg of caffeine per day. This is the equivalent of approximately 3 cups of instant coffee, or one and a half cups of brewed coffee. The same intake can be achieved by drinking 3 cups of tea, 5 glasses of coca cola or 2 cans of Red Bull. Caffeine is also found in a range of energy drinks and other consumables including chocolate.
What are the limitations of the studies?
Whilst this large meta-analysis suggests a possible link between caffeine intake and miscarriage and low birth weight, scientists remain divided over the risks. The study was unable to rule out the possible influence of other factors such as maternal age, smoking or alcohol use. Many studies, such as a recent article by Savitz et al (2008) have been unable to find any significant ill-effects of caffeine intake during pregnancy. It is unethical to perform a placebo-controlled investigation, so the evidence available to scientists is limited.
What advice is given to pregnant women?
In light of the available evidence, the NHS recommends pregnant women should limit their caffeine intake to no more than 200mg per day (NHS Choices website). However, women should discuss any particular individual concerns with their own GP or midwife.
“Caffeine in the diet” Medline Plus website
R Knutti, H Rothweiler, C Schlatter “Effect of Pregnancy on the Pharmmacokinetics of Caffeine” European Journal of Clinical Pharmacology 21:121-126
O Fernandes, M Sabharwal, T Smiley “Moderate to Heavy Caffeine Consumption During Pregnancy and Relationship to Spontaneous Abortion and Abnormal Fetal Growth: A Meta-Analysis” Reproductive Toxicology 12:435-444
“Caffeine Content of Drinks” Energy Fiend website.
DA Savitz, RL Chan, AH Herring “Caffeine and Miscarriage Risk” Epidemiology 19:55-62
“Which Foods Should I Avoid During Pregnancy?” NHS Choices website.
Disclaimer: The information contained in this article is for educational purposes only and should not be used for diagnosis or to guide treatment without the opinion of a health professional. Any reader who is concerned about his or her health should contact a doctor for advice.