Many hospitals across England had to turn away women in labour last year because they were full, figures show.
Some maternity units frequently have to turn women away
More than 40% of 103 trusts that responded to a survey by the Tories said they had shut their doors or diverted women to other sites.
Larger maternity units seemed to be more at risk of having to close, the inquiries by the Tories suggested.
The government said maternity units sometimes were forced to take action because it was hard to predict demand.
Labour's manifesto statement says that by 2009 all women will have choice over where and how they have their baby.
It also promised that every woman would be supported by the same midwife throughout her pregnancy.
Out of 103 trusts - 70% of the total - providing maternity services that responded to the freedom of information request, 42% had to close their units or divert women to another site at least once in 2007 because of capacity problems.
One in 10 said they had shut their doors more than 10 times.
University Hospitals of Leicester NHS Trust, one of the biggest maternity providers in England, reported closing 28 times.
Of those trusts that had to turn women away, 74% had more than 3,000 births last year, the Conservatives said.
Shadow health secretary Andrew Lansley said: "The government's plans to close maternity units when services are already overstretched fly in the face of common sense.
"Labour are fixated with cutting smaller, local maternity services and concentrating them in big units.
"But women don't want to have to travel miles to give birth.
"And they certainly don't want to have to travel even further because they're turned away by the hospital of their choice."
A spokesperson for the National Childbirth Trust said: "Parents lose all sense of control when they are told that the unit at which they have planned to give birth has been closed temporarily, at short notice.
"It is a major cause of anxiety to telephone, or even arrive at a maternity unit, when in labour to find the doors are shut.
"Unscheduled closures should only occur in very exceptional circumstances when to keep a unit open would be unsafe."
A Department of Health spokesperson said diverting women to other hospitals should be the exception rather than the rule.
"It is difficult to precisely predict when a mother will go into labour and sometimes, at times of peak demand, maternity units do temporarily divert women to nearby facilities.
"When this does happen it is often only for a few hours and to ensure mother and baby can receive the best care possible."
Sally Russell, director of Netmums said: "This puts women in a state of anxiety around childbirth at a time when they want everything working in the right way.
"There's a tendency for PCTs to think of birth as a one-off occasion but it affects mothers and children for a long time to come so it's important to get this right."
Louise Silverton, deputy general secretary of the Royal College of Midwives, said women should have a choice of how and where they give birth.
"If a unit closed because of staff shortages, then their choice is eroded and their experience of birth suffers."