Researcher Dr Katherine Laughon put the increase down partly to the differences in the women. The modern-day mothers weighed more before and during pregnancy and were, on average, around four years older than those in the 1960s group.
And more than half of the contemporary women had epidurals - which can prolong labour by between 40 and 90 minutes - compared with four per cent of 1960s deliveries.
In addition, women were usually confined to bed after an epidural, which may also slow down labour.
But Dr Laughon, writing in the American Journal of Obstetrics and Gynecology, said she could not fully explain the difference in labour times.
For example, changes in medical practice over the past 50 years - such as using the drug oxytocin to speed up labour - should lead to shorter delivery times. Dr Laughon said the lengthening of labour means doctors may be able to wait longer before intervening with oxytocin or delivering the baby by caesarean section.
- DAILY MAIL