Women who drink tea or coffee during pregnancy are more likely to have small babies, research suggests.
Even women who drink less than the ‘safe’ cutoff of 200mg caffeine – about two mugs of instant coffee or three cups of tea – are more at risk of having low-birth weight or premature newborns.
The researchers, from University College Dublin, believe caffeine restricts blood flow to the placenta, affecting babies’ growth. The study, led by doctor Ling-Wei Chen, looked at 941 mother-child pairs born in Ireland. Tea was the mothers’ main source of caffeine (48 per cent), followed by coffee (38 percent).
The results suggested that for every additional 100mg of caffeine – around half a cup of coffee – consumed daily during the first trimester, birth weight was reduced by 0.5lbs (72g).
This amount of caffeine also decreased the babies’ length and head circumference, as well as their gestational age, which measures the length of pregnancy.
Results further found the women who consumed the most caffeine had babies weighing around 0.37lbs (170g) less than those who had the least.
Even women who took in less than the ‘safe’ amount of 200mg of caffeine, saw significant affects. This is amount is deemed safe by the British National Health Service (NHS).
The study was published in The American Journal of Clinical Nutrition.
Chen told Reuters: “Based on the consistent associations we observed, and because many pregnancies are unplanned, we would recommend women who are pregnant or seeking to become pregnant at least limit their intakes of caffeinated coffee and tea.
“High caffeine intake can result in restricted blood flow in the placenta which may subsequently affect fetal growth.
“Caffeine can also cross the placenta readily, and because caffeine clearance slows as pregnancy progresses, caffeine accumulation may occur in fetal tissues.”
The researchers worry people are unaware of how much caffeine their tea contains. Tea has less caffeine than a cup of coffee, but the exact amount depends on the brew time, water temperature and type of tea.
The Department of Nutritional Services reports a cup of black tea contains anywhere between 23 and 110mg of caffeine.
The World Health Organization recommends women consume less than 300mg of caffeine a day during pregnancy. Whereas the NHS and American College of Obstetricians and Gynecologists advise no more than 200mg.
However, the most recent research suggests this is too high, according to Dr. De-Kun Li, a scientist at the Kaiser Permanente Northern California division of research in Oakland.
He was not involved in the study, but found a link between caffeine consumption in pregnancy and miscarriage in a 2015 study.
“Epidemiological findings based on self-reported caffeine consumption are usually not very precise. Thus, any cutoffs chosen by ACOG and WHO can only be considered as rough reference points,” Li said.
“Biologically, it is unlikely that 300mg is risky while 299mg is safe. The message to women I would prefer would be ‘the less the better’.
“My advice would be try to reduce as much as you can, if you can totally quit that would be even better.”
ACOG added it reviews all of its recommendations every 18-to-24 months, includes all new research in its reviews and makes adjustments to recommendations as needed.
In 2008, a trial of 1,063 pregnant women in San Francisco showed that those who consumed at least 200mg of caffeine every day have a 25 per cent risk of miscarriage compared with a 12 per cent risk for those who avoided the stimulant, which is also found in soft drinks and chocolate.
Another study found caffeine consumption during pregnancy appears to promote childhood obesity.