Call to lock up fewer mothers.
The figure comes 18 months after the Government-commissioned Corston Report recommended custody be a last resort for women, particularly those caring for children.
Lib-Dem home affairs spokesman Chris Huhne accused Labour's efforts to "look tough" on crime for a doubling in the number of women in jail over the past decade.
He said fewer women should be locked up, and those who were should be kept in small units close to their homes.
Information provided by justice minister Maria Eagle in response to a Parliamentary question from Mr Huhne showed that there were 50 children aged under two in mother and baby units on November 30 - the last date for which figures were available.
The prison system in England and Wales has places for 75 mothers with babies, including Styal in Cheshire, which takes them up to 18 months of age, or on occasions two years.