HUDDERSFIELD women may increasingly opt to give birth at Halifax in the wake of the health changes, an expert has warned.
Professor Jim Thornton, a consultant obstetrician based at Nottingham City Hospital, is against the changes voted through last week.
Speaking to the Examiner, Prof Thornton said: "What's genuinely worrying is a free-standing midwife led unit in a large town or city.
"My prediction is that the free-standing midwife led unit in Huddersfield will have fewer women delivering in it than they expect.
"There's no epidural service, no forceps or caesarean. I would suspect that it will not have as many women as the planners hope.
"I suspect, I don't really know, but the women of Huddersfield will say: "Give me all the facilities please," and go to Halifax."
He said plans to make health services more efficient could backfire if less women give birth in Huddersfield.
"There will be some days where there will be no births at all and you're staffing an empty ward.
"There's evidence that previous trust mergers when looked at in detail have failed to deliver as much cost savings as they hope for when they started."
Prof Thornton was author of a report on maternity services in the UK, for the group Doctors For Reform.
Local campaigners spotted his work and asked him to assist their protests.
Prof Thornton talked to Kevin Holder and other health chiefs.
"When I looked at their reports I felt that they were exaggerating the costs of keeping two maternity units. I think some of my arguments they took on board, but they didn't agree with all of them."
He said he was against the merger because it would not improve patient choice.
There are around 2,700 births at the HRI a year. The merged unit will cater for between 4,000 and 5,000.
He added: "The units at the moment are not small, they're just over the average size.
"But by European standards they would be very big units.
"The largest unit in Belgium is 2,400, the largest in Germany is just over 3,000, the largest in France is Lille and it's just over 4,000."
As for the risks he said: "I think it's a real concern. There must be some risks.
"There's always going to be someone halfway through labour where something totally unexpected happens. Every obstetrician accepts that."
Prof Thornton added: "I hope that Patricia Hewitt, who speaks so strongly in favour of patient choice, will put a stop to this."