Pregnant women should avoid drinking coffee because even a single cup of it a day could harm their unborn babies, researchers have warned.

They reported in the journal BMC Medicine that caffeine content is linked to low birth weight and prolonged pregnancies, according to the Daily Express.

The researchers used information about almost 60,000 pregnancies over 10 years.

They found that caffeine reduced birth weight by 21g to 28g per 100mg consumed a day by mothers.

Caffeine from all sources also increased the length of the pregnancy by five hours per 100mg a day.
But caffeine intake specifically from coffee was associated with eight hours extra for every 100mg per day consumed.