The maternity unit in Bridgend's Princess of Wales Hospital will remain closed for the rest of the year.
Refurbishment work, which is also affecting the neonatal wards, was due to be completed within twelve weeks, with services moved to other hospitals across South Wales.
But the Cwm Taf Morgannwg health board says it must now carry out urgent work to replace the roof of the building, after safety concerns were raised.
They've been unable to give a confirmed date for reopening.
Earlier this week, Bridgend councillors were told the additional repairs would cost around £20 million after part of the roof had disintegrated.
In a statement, a spokesperson for Cwm Taf Morgwannwg said "an urgent multi-million programme of work" was needed to replace the roof on the main building, including the area where our maternity and neonatal wards are located.
They added: "While the roof in this area will be prioritised and be the first to be replaced, the work will unfortunately mean that our maternity and neonatal services will not be able to move back to the hospital in early December as planned."
"This is an urgent and fast moving situation, and we do not have a definite date for return, however we anticipate this to be in early 2025. We commit to keeping you informed of dates and timescales as soon as they become available."
The health board added that the refurbishment work - described as "urgent and essential" - was currently on track.
Since the closure, expecting mothers in the Vale who chose to give birth at the Bridgend hospital have been allowed to choose an alternative unit - including the University Hospital of Wales (UHW) in Cardiff, the Prince Charles Hospital in Merthyr or Singleton Hospital in Swansea.
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