The Royal College of Midwives in Wales has expressed caution about plans to close a midwife-led maternity unit.
It recognised the NHS cash pressure but is concerned about "hasty decisions".
Cardiff and Vale University Health Board (UHB) wants to move the service from Llandough Hospital to the University Hospital of Wales.
The board said only one baby a day was born at Llandough and it was important to consider "how best to use our staff and our facilities".
The health board said its plans had not yet gone out to public consultation.
Many health officials across Wales are reviewing their maternity provision as they work to meet financial targets.
Cardiff and Vale UHB has proposed closing the six-bed midwife-led unit Llandough Hospital, which opened in 2005.
The plans are put forward in its document, Making a difference, by the health board's director of planning.
The service would transfer to the city's main hospital, which has a larger midwife-led unit as well as a consultant-led service.
Women would still be able to opt for a home birth.
Helen Rogers, head of the Royal College of Midwives in Wales, said she had asked for assurances that there was enough capacity at the University Hospital of Wales.
She said: "It is a reduction in choice.
"We are aware of the financial pressures the health board is under, however, a reduction in what is an important service is not necessarily the way to respond to those pressures.
"What I would not want to do is to rush into decisions that may be short term decisions which may be to the detriment of the service and the women in the longer term.
"I'm concerned about hasty decisions that might not be to women's benefit in the long term.
"This isn't just a Cardiff issue, it is an all-Wales issue. If there is any change in the service provision around the rest of Wales, it could have an impact on Cardiff as well."
Welsh Conservative health spokesman Andrew Davies said he had "strong reservations" about the transfer after a meeting with the health board director of planning and the head of midwifery and gynaecology nursing.
He said: "I expressed these [reservations] along with many of the concerns held by local constituents who have been in touch with me in recent days."
Ruth Walker, executive director of nursing at Cardiff and Vale UHB the proposal had to be set in the context of choice for pregnant women and providing high standards of care.
"The UHB recognises it is important that parents have access to a full range of maternity services that can properly meet their needs at one of the most important times in their lives," she said.
"We know that many women want to give birth in a midwifery-led unit and this proposal in no way weakens our commitment to the midwifery-led model of care or reduces choice.
"Women will still be able to choose where to give birth, for example at home, at UHW in either a midwife-led or consultant-led unit."
She said ante-natal and post-natal care would continue to be provided locally.
"Given that only around 6% of babies born in Cardiff and the Vale (one per day) last year were delivered at Llandough, it is important that we consider how best to use our staff and our facilities to provide the best possible service to parents and their babies."
She said this was particularly true when a new £16m women's unit at UHW is near completion, which included a significant investment in recruiting more midwives.
Ms Walker said the plans were being discussed with staff and key stakeholders, to ensure the decision was based on "sound clinical and professional advice".
"We will play our full part in any national discussions on the future of maternity services across Wales and have already talked to our colleagues in neighbouring health boards about our plans for Cardiff and the Vale," said Ms Walker.