The Royal College of Midwives (RCM) has accused the government of backing down on a pre-election pledge to increase midwife numbers in England.
It says mothers and babies will suffer unless the 3,000 extra midwives it says were promised are delivered by 2014.
RCM general secretary Cathy Warwick will tell its annual conference there are too few to cope with a rising birth rate and increasingly complex births.
The Tories said the rise was now not needed as the birth rate was stable.
Speaking ahead of Wednesday's conference in Manchester, Ms Warwick told BBC Radio 5 live ministers had gone back on their word.
She said: "Just before the election, both the prime minister and the deputy prime minister told us that they would commit to continuing the previous government's promise to give us more midwives.
"We've just done a survey of all the heads of midwifery and they've got vacant posts but they're having difficulty filling them."
The RCM survey of 83 out of 194 heads of midwifery suggested maternity units were already seeing budget and staffing cuts.
Some 30% said their budgets had fallen over the past year, while a third had been asked to reduce staff.
And two-thirds surveyed said they did not have enough midwives to cope with demand.
Meanwhile, the number of live births in England has risen by 19% between 2001 and 2009, to 670,000 a year.
The RCM said births over the same period had become more complex, with obese pregnant women and older or teenage women needing extra support.
The Royal College of Obstetricians and Gynaecologists (RCOG) said maternity services were facing many challenges.
President, Dr Tony Falconer, said: "As well as need for more midwives, there is a need for more consultants to deal with the increase in the number of high-risk pregnancies."
A Conservative Party spokesman said: "The commitment to 3,000 midwives made in opposition was dependent on the birthrate increasing as it has done in the recent past. It was not in the coalition agreement because predictions now suggest the birthrate will be stable over the next few years.
"People can be absolutely clear that our commitment to meet the needs of expectant mothers remains, and we will continue to train new midwives to meet the demands arising from the births."