The maternity unit at Bridgend's Princess of Wales Hospital will be closed for the next twelve weeks.
The neonatal unit has also been shut to allow for "urgent and essential" refurbishment work to take place.
Expecting mothers in the Vale who have chosen to give birth at the Bridgend hospital will be able 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.
Tirion Birth Centre, the free-standing midwifery unit based at the Royal Glamorgan Hospital in Llantrisant, is also available.
The Cardiff and Vale University Health Board says anyone under their care will not have their treatment affected by the temporary closure.
But the maternity unit at UHW is preparing to accommodate an extra 40 to 70 births each month during the closure period, with staff from the Cwm Taf Morgannwg health board being relocated to the Welsh capital.
A spokesperson for CAVUHB said: "The maternity team has been working closely with CTM to make sure there are appropriate staffing levels to safely accommodate the increase in births at UHW."
"Providing safe, effective and compassionate care to all women and birthing people is always our priority and will continue to be so during this time."
"Your antenatal and postnatal care will also continue as normal."
More information affecting patients in Cardiff and the Vale can be found via CAV Maternity Services on Facebook or the CAVUHB website.