Some maternity units are struggling to cope as the birth rate rises, according to a new report.
Last year there was one baby born in the UK every 40 seconds - the highest number for nearly 20 years, BBC1's Panorama programme reports.
Across the UK, maternity units were forced to close temporarily to new admissions 1,055 times last year, nearly always because of under-staffing or lack of beds, which meant at least 927 women had to be turned away, the programme said.
It carried out a survey using the Freedom of Information Act - 171 Health Trusts and Boards were contacted, and almost 90% replied.
Cathy Warwick of the Royal College of Midwives told the programme: "To turn up at the place you were expecting to have your baby and be told 'well I'm sorry you can't come here, you've got to go down the road to a different unit', I just think that's not a high standard of care and it should not be what we're giving women of this country."
The survey found that the average midwife vacancy rate in Wales and Northern Ireland as of May 1 2010 was below 1% and in Scotland just above 1%. The English vacancy rate was nearly 5%. But in some London trusts the rate was as high as 19 or 20%, meaning that one in five posts was lying vacant.
The programme was given exclusive access to an as yet unpublished independent inquiry - commissioned by NHS London - into a spike in maternal deaths in the capital. The review looked at 42 deaths of women over an 18-month period from January 2009 and found that substandard treatment was a major factor in 17 cases.
The report found some maternity units struggling to cope and said: "Escalation of services was noted to be an issue when units appeared to have difficulty coping with the clinical workload; several deaths occurred when activity in a maternity unit was high and one to one care could not be delivered".
In a statement, Health Minster Paul Burstow told the programme: "Safety is paramount in the NHS and all mothers and their babies should expect and receive consistently excellent maternity care. There is a record number of midwives entering training this year and the number of consultants in obstetrics and gynaecology has increased from two years ago.
"We will continue to work with the Royal College of Midwives to make sure we have an appropriately resourced and skilled maternity workforce based on the most up-to-date evidence."
