The Conservatives claim maternity services are "in chaos" because midwives are being put under "intolerable pressure" by having to deliver almost 25% more babies than experts say they should.
The government's pledge to offer women a choice about where to give birth by 2009 is in danger of failing because of "woefully inaccurate" birth rate estimates, the party said.
Estimates of the number of live births are now out by around 40,000 after a huge increase in fertility rates over the last few years, it added.
The Tories are warning that every midwife now delivers almost 25% more babies each year than the Royal College of Midwives recommends.
The number of live births has increased by 12.5% since 2001, while the number of midwives has increased by only 4.5%, figures released by the party show.
Shadow health secretary Andrew Lansley criticised the government, saying: "Their flagship policy to offer choices to expectant mothers over where and how they give birth is underpinned by assumptions about the number of births which are wide of the mark.
"Mothers want choices over the care they receive during pregnancy and high standards which depend upon the availability of midwives, yet Labour's current plans cannot deliver it."
But a Department of Health spokesman said: "There are now 2,423 more midwives than there were 10 years ago.
"The number of students entering the profession has risen by 44%. And 80% of women are happy with the maternity services they receive. This is hardly a picture of 'chaos'."