Coffee has been linked to low birth weight babies and longer pregnancies.
The caffeine passes the placental barrier but the developing baby does not have the necessary enzymes to inactivate it efficiently.
The World Health Organisation currently recommends a limit of 300mg of coffee a day during pregnancy but some countries recommend less than a single cup from some high street cafes.
A Norwegian study of almost 60,000 pregnancies over 10 years looked at all sources of caffeine taken by the mothers from coffee and tea to fizzy drinks as well as food with cocoa like cakes, deserts and chocolate.
They found that caffeine from all sources reduced the baby’s birth weight by about 21-28g per 100mg of daily caffeine.
Caffeine from all sources increased the length of the pregnancy by five hours per 100mg of daily caffeine.
But caffeine from coffee increased the pregnancy even long – by eight hours for every 100mg daily.
According to the researchers “this association means that it is not just the caffeine in coffee which increases gestational length but either there must be a substance in coffee which is responsible for the extra time or there is a behaviour associated with coffee drinking not present in women who drink only tea (for example). “
They point out that smaller birth weight babies are at higher risk of both short term and lifelong health problems and it seems from the results of the study that even 200-300mg caffeine per day can increase the risk of SGA by almost a third.
The findings are published in BioMed Centrals’s open access journal BMC Medicine.