Maternity services at Daisy Hill Hospital have been temporarily suspended due to staffing shortages.
As a result of "unexpected sickness", the Southern Health Trust said it is "unable to provide safe medical cover" in obstetrics and gynaecology at the Newry site this weekend.
The changes came into effect on Friday evening and will remain in place until 09:00 BST on Monday.
During this period, all obstetric and gynaecology care will be provided at Craigavon Area Hospital.
A small number of maternity patients in Daisy Hill have been transferred to Craigavon to continue their care.
The Southern Trust said the decision was taken to ensure the safety of women and babies.
It apologised for any concern or inconvenience caused and said the temporary move forms part of its service continuity arrangements when staffing pressures arise.
The trust said it "continues to face ongoing medical staffing challenges" within obstetrics and gynaecology at Daisy Hill.
Women who need advice have been asked to contact the Maternity Assessment Unit at Craigavon Area Hospital on 028 3756 1830.
The trust added that the situation is being kept under review.
The chair of the British Medical Association's (BMA) Northern Ireland council has described the temporary suspension as "an inevitable consequence" of staff shortages.
"To be able to maintain current services we need to attract new staff and retain current staff by valuing them properly for the work they do," Dr Alan Stout said.
"Clearly the system is failing at addressing this issue."
Sinn Féin MLA Liz Kimmins told BBC News NI that she has been engaging with senior representatives within the Southern Health Trust and it was her understanding that it's "due to a lack of staffing cover which left it unsafe essentially for expectant mothers coming in".
She said it is an "unacceptable" situation, but added that there are a number of jobs that need filled that make it "very challenging".
"We have recently met the trust, we were aware that there are vacancies that they are actively recruiting for, and I think when situations like this arise, where the only cover they have is unable to come in and it was due to sickness, we realise just how critical it is that we get this service stabilised."
She said it will be a "challenging weekend" for the staff in Craigavon Hospital, but wanted to assure the public that it's a "temporary measure".
DUP assembly member (MLA) Diane Forsythe said the temporary closure "has understandably caused serious concerns for staff and patients".
"I have been in contact with the management of the trust and Department of Health overnight escalating concerns," she said in a statement.
"There are a number of questions needing answered on this which I will follow up on."
She added there "needs to be clarity and measures in place so this does not happen again".
Justin McNulty, from the SDLP, said the closure is causing "dismay" among midwives and mothers, who he said "deserve better".
The MLA added that he has sought an urgent meeting with the chief executive of the Southern Trust "seeking reassurance that the appropriate actions will be taken to ensure this does not happen again".
