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.

Barry Christmas events to go ahead with extra security following town centre disorder
Police identify 50 young people amid efforts to tackle Barry town centre disorder
Reform MS calls for “no more devolution in any area”
Barry museum to capture local Gavin & Stacey memories in new video project
Extraordinary council meeting called over anti-social behaviour in Barry town centre
Local Dinas Powys pupils gain insight into gas safety
Former Holm View Leisure Centre could become education and community hub
ScoutsCymru members represent Wales at Royal Albert Hall Festival of Remembrance
Plans for indoor market at former Barry Wilko move forward
Senedd passes supplementary budget with £60m boost for additional learning needs
Consultation to open on plans for 35MW solar farm near Penarth
No Trains through the Vale of Glamorgan this Sunday
Police and Cardiff City launch operation to tackle matchday anti-social behaviour
Free leaf and fallen fruit collections return in the Vale
Barry retains Fairtrade Town status until 2029
£1m improvements planned for Pencoedtre Park as splashpad work set to begin
Plan submitted for new independent school on site of well-known charity farm
Is your hot water bottle safe?