More than two mugs a day coffee linked to birth of smaller babies
Coffee may also make pregnancy last longer – but only by a matter of hours.
The observations come from researchers who studied detailed records of almost 60,000 pregnancies from a ten-year period in Norway.
The records included information about how often the women had foods or drinks containing caffeine, from tea and coffee to chocolate sandwich spread and bars of chocolate.
Comparing this with details of their baby revealed a clear link with caffeine and birth weight, with 200 to 300mg a day raising the odds of the child being classed as small for the length of the pregnancy by up to 62 per cent.
A mug of instant coffee contains around 100mg of caffeine, and a mug of filter coffee, around 140mg of caffeine.
However, some drinks sold in high street coffee shops contain as much as 300mg per cup.
In Britain, as in Scandinavia, pregnant women are advised to limit their caffeine intake to 200mg a day.
Coffee, specifically, was found to be linked with increasing the length of pregnancy, with a daily mug of instant coffee lengthening the time the baby is in the womb by eight hours.
Unlike some previous studies, the research did not make a link between caffeine and premature birth.
Researcher Dr VerenaSengpiel, of the Sahlgrenska University Hospital in Sweden, said that caffeine may hurt the growth of the unborn baby by slowing the vital passage of nutrients from mother to baby via the placenta.
Writing in the journal BMC Medicine, she added that coffee may make increase the length of pregnancy by a matter of hours by interfering with the chemical signals that occur around the onset of labour.
Dr Euan Paul, of the British Coffee Association, said: ‘The UK Food Standards Agency carefully analysed and thoroughly reviewed the effects of caffeine during pregnancy and currently recommends that pregnant women moderate consumption to an upper safe limit of 200mg / day – two to three cups of coffee.
‘Switching to decaf during pregnancy is also an option for those who wish to continue drinking coffee.
‘We welcome more research into this important area so that the associations found in this study can be further explored.’
Annette Briley, consultant midwife for the baby charity Tommy’s, said: ‘Being born small can lead to catch-up growth and this in turn can lead to obesity, diabetes and certain cancers in adult life.
‘While women do need to be mindful and remember that caffeine is found in tea, chocolate, other sweets and soft drinks – as well as coffee – we would suggest further research into the effects of coffee is required.
‘Additional care should however be taken when buying coffee in retail outlets as the caffeine content varies between many companies. If women are worried, they should seek advice from their GP or Tommy’s midwives for the best advice.’
In 2011, Glasgow University researchers warned that pregnant women could unwittingly be putting their the health of their unborn baby at risk by drinking coffee from high street cafes.
Their analysis of espressos from 20 coffee shops found huge variations in the amount of caffeine, with the strongest having more than six times as much as the weakest.
Drinks from four cafes had more than the 200mg daily limit recommended for pregnancy and one coffee contained more than 300mg.