Many hospitals across England had to turn away women in labour last year because they were full, figures show.
Some 70% of trusts provided data, with more than 40% saying they shut their doors - one in 10 more than 10 times - 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.
Out of 103 trusts 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.
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