The study has shown that any amount and type of caffeine intake—from tea, cola, chocolate, cocoa, and some prescription drugs, as well as coffee—is linked with relatively slower fetal growth.

To reach the conclusion, Dr Justin Konje and colleagues from the University of Leicester and collaborators from the University of Leeds, examined the association of maternal caffeine intake and individual caffeine metabolism on birth weight, reports the British Medical Journal.

The boffins recruited 2645 low risk pregnant women of average age 30, who were between 8-12 weeks pregnant from two large teaching hospitals in the UK between September 2003 and June 2006.

The research team used a caffeine assessment tool (CAT) to record caffeine intake from all possible dietary sources in the four weeks before and throughout pregnancy, and also used a saliva sample test to calculate individual caffeine metabolism.

The researchers report that the average caffeine intake during pregnancy was 159mg/day, much lower than the limit of 300mg/day recommended by the UK government's Food Standards Agency.

Interestingly, 62 per cent of the caffeine use reported came from tea. Other sources were coffee (14 per cent), cola (12 per cent), chocolate (8 per cent), and soft drinks (2 per cent).

Most of the babies were born at full term, with an average birth weight of 3450g, while 4 per cent were born prematurely, 0.3 per cent were stillborn, and 0.7 per cent were miscarried late. Overall, the results confirmed that these were low risk pregnancies.

However, the authors found a ''dose-response relationship'', showing that increasing caffeine intake was associated with increasing risk of fetal growth restriction (FGR).