The U.K. findings, published in the online journal BMJ Open, have prompted the researchers to query whether mums-to-be should cut caffeine out of their diet altogether.
Caffeine passes rapidly through tissues, including into the placenta, and it takes the body longer to get rid of it during pregnancy. It has been linked to a heightened risk of miscarriage and restricted foetal growth.
In the study, children exposed to very high levels of caffeine before birth weighed 67-83g more in infancy (3-12 months); 110-136 g more as toddlers; 213-320g more as pre-schoolers (3-5 years); and 480g more at the age of eight than children who had been exposed to low levels.
"Maternal caffeine intake may modify the overall weight-growth trajectory of the child from birth to eight years," the authors wrote. "The results add supporting evidence for the current advice to reduce caffeine intake during pregnancy and indicate that complete avoidance might actually be advisable."
The researchers drew on just under 51,000 mother-and-infant pairs, all of whom were part of the Norwegian Mother and Child Cohort Study between 2002 and 2008. At 22 weeks of pregnancy, the mums-to-be were asked to quantify their food and drink intake from among 255 items, including caffeine, using a specially adapted Food Frequency Questionnaire.
Sources of caffeine included coffee, tea, caffeinated soft/energy drinks, chocolate, chocolate milk, sandwich spreads, and desserts, cakes, and sweets. Daily intake was grouped into: 0-49mg (low); 50-199mg (average); 200-299mg (high); and 300+mg (very high).
Their children's weight, height, and body length were subsequently measured at 11 time points: when they were six weeks old; at ages three, six, eight and 12 months. And then at one-and-a-half, two, three, five, seven and eight years of age.
Just under half of the mums-to-be (46 percent) were classified as low caffeine intake; 44 percent as average intake; 7 percent as high; and 3 percent as very high.
The higher the intake of caffeine, the greater the likelihood that the mother was older than 30, had more than one child, consumed more daily calories, and smoked during her pregnancy. And women with a very high caffeine intake during their pregnancy were more likely to be poorly educated, and to have been obese before they got pregnant.
"Average", "high", and "very high" caffeine intake during pregnancy were associated with a heightened risk — 15 percent, 30 percent, and 66 percent, respectively — of faster excess growth during their child's infancy than low intake, after taking account of potentially influential factors.
Exposure to any caffeine level while in the womb was associated with a heightened risk of a child being overweight at the ages of three and five years, although this persisted only for those eight-year-olds whose mums had had a "very high" caffeine intake during their pregnancy.
The authors said this was an observational study, so it can't confirm causality, while questionnaires can only provide a snapshot in time of dietary behaviour. Nevertheless, the researchers point to the large sample size, the consistency of their findings, and a plausible biological explanation — foetal programming.
In March 2018, a study revealed that pregnant woman were not aware what the recommended 200mg of caffeine per day actually looked like. In a poll of 4,100 pregnant women, 61 percent said they would reduce their caffeine consumption habits after being made aware of how much caffeine there is in daily items.
200mg is equal to...
:: Two mugs of tea (350ml) a day - 75mg each
:: One mug of filter coffee - 140mg
:: Two mugs of instant coffee - 100mg each
:: Five cans of cola - 40mg each
:: Eight plain chocolate bars (50g) - 25mg each (FYI milk chocolate has less caffeine than dark chocolate)
:: Two energy drinks (250ml) - 80mg each.