NHS Wales' medical director admits that some smaller maternity units in Wales are not "safe or sustainable".
Dr Chris Jones said the units were dependent on temporary agency staff at a cost of millions of pounds a year.
He said the NHS had no way of ensuring the quality and competence of these staff or their immediate availability.
The assembly government said his "honest and transparent" comments reflected the views of the Royal College of Obstetricians.
They come in the wake of controversial proposals to review maternity services in north Wales, being looked at by Betsi Cadwaladr University Health Board.
Dr Jones said a number of maternity units in Wales were not delivering enough babies to attract doctors because they did not meet training guidelines for out-of-hours deliveries.
This led to an increase in vacancies at the maternity units, he said.
He said: "Now, although all our rotas are European Working Time Directive-compliant on paper, we know in practice they are only being maintained by significant expenditure.
Medical Director, NHS Wales
"All of our health boards are spending hundreds of thousands of pounds in locum costs, agency costs, extra payments to doctors, to maintain these rotas.
"Betsi Cadwaladr health board is spending up to a million pounds a year to maintain their rotas.
"The difficulty is that this is not a safe or sustainable service, because you cannot always guarantee you'll get someone to fill a shift.
"If you do get someone, you can't always guarantee their experience or their competence.
"We know there are risks with peripatetic locums, and we know that this risk is increasing rapidly.
"So we realise that this is a major area of risk for these services as currently configured, and they will not be sustainable in the way they are configured in the future."