Many hospitals across England had to turn away women in labour last year because they were full, figures show.
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.
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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
