Figures, compiled by BBC Radio 4's Woman's Hour, suggest that a quarter of NHS Trusts have not assessed their midwife workforce needs for at least four years, while almost one in 10 have not done an assessment in the last decade.
The Royal College of Midwives (RCM) has said senior hospital staff may have been avoiding these assessments because they knew they could not afford to fill any staffing gaps, which might be exposed.
The figures also showed that 80 per cent of NHS trusts still had vacancies for midwives.
"Four in every five NHS trusts say they have midwife vacancies, a situation that we feel is getting worse, not better," RCM chief executive Cathy Warwick said. "This has to change."
The demand for hospital maternity services has grown in recent years, as according to the latest statistics, the annual number of births in England has increased by a quarter in the past decade.
"The pressure on maternity units becomes worse when there is a mix of insufficient midwives to begin with," said Cathy. "On top of this, midwifery vacancies are not being filled.
"This is a recipe for disaster and can have a disastrous impact on staff morale, burnout and sickness rates, which only make a maternity service even more short staffed.
"We hear from some heads of midwifery that trusts are not conducting proper assessments of staffing requirements because they know they won't be able to afford to implement the findings.
"So they bury their heads in the sand and stick with their out-of-date assessments that no longer bear relation to their needs and the needs of mothers and babies."
Of the 99 trusts that responded to a freedom of information request, 24.2 per cent had made no calculation of their needs in the past four years and 66.7 per cent hadn't done so in the past two years.
A further nine per cent had done no assessment for at least a decade.
"Our assessment is that we need around 4,800 more midwives in England," added Warwick. "The Government is increasing the number of midwives, and that is welcome, but more needs to be done."
Health minister, Dr Dan Poulter, said: "The NHS is a safe place to give birth, with women reporting high levels of trust and confidence in staff."
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