Inpatient maternity services at Daisy Hill hospital in Newry will be temporarily suspended from 23 November.
Southern Health Trust board members have voted in favour a proposal to move the services to Craigavon hospital, as the trust says it has been unable to recruit enough consultant obstetricians to safely staff both hospitals.
There has been significant opposition to the consolidation plan from people in Newry and further afield, including South Down and South Armagh - and it's now subject to a legal challenge.
A 40-year-old pregnant woman is to lodge judicial review papers at the High Court on Tuesday afternoon to attempt to force the trust to delay the implementation of the plan.
While the move has been described as temporary, there is currently no time-frame on the restoration of services at Daisy Hill.
The Trust has said it does not want to be in this position but it has exhausted all recruitment attempts.
It needs eight substantive consultants - from the end of this month it will have two, plus three locum consultants.
Questions have also been asked about Craigavon's ability to cope with the additional 1,700 births expected there, as a result of the change.
The Trust said Craigavon is ready to deal with the increase in demand.
Steve Spoerry, Chief Executive of the Southern Health and Social Care Trust, has described the temporary suspension of services as a "sad day".
Spoerry also apologised to the community, but said there was no other alternative.
He said the Trust "cannot continue to safely provide inpatient maternity care at both hospitals".
"We will continue our work to build a real viable future for Daisy Hill as a general hospital serving its population," Spoerry said.
"We will continue with the antenatal and postnatal services, we will continue on our work to build a real viable future for Daisy Hill as a general hospital serving its population."
Spoerry added that there are preparations for women attending hospital unexpectedly.
Every woman will have a designated midwife when they register for birth, while the ambulance service will be able to triage women to Craigavon Hospital or elsewhere as appropriate.
'The right thing to do'
Dr Beverly Adams, Divisional Medical Director for Maternity and Women's Services, said a centralised telephone triage service with a midwife would help determine the most appropriate place for each woman to give birth.
"If they're of a higher risk or there's a concern about being in labour then advising them to attend Craigavon instead is the right thing to do," she said.
She acknowledged that women in more rural areas, particularly South Down and South Armagh, had already been advised in some cases to give birth at Craigavon or the Royal Victoria Hospital.
"We're used to planning around that and advising those women in that regard.
"If they're of a higher risk or there's a concern about being in labour then advising them to attend Craigavon instead is the right thing to do."