The two-star Royal Bolton hospital has commissioned an independent inquiry after three women died in childbirth in its maternity unit between October 2005 and the middle of this month. The baby of one of the mothers also died.

Internal investigators have found no link, but because deaths in childbirth are so rare it requested the second inquiry, which will report at the end of February. Before October, only one woman had died at the unit since 2000. Fewer than 90 mothers die nationally in childbirth each year, with 13.1 maternal deaths occurring for each 100,000 births.

Yesterday, a spokeswoman for Bolton Hospitals NHS trust stressed there was no obvious link between the deaths, which occurred with different teams of staff.

"The trust has been holding investigations into each case and so far can find no evidence that the fact that these three cases came so close together was anything other than a tragic and unforeseeable coincidence. However, because such cases are so very unusual, the trust itself has commissioned an external independent review so it can see if there are any lessons that can be learned," she said.

Gail Naylor, the head of midwifery services, said staff had been upset. "They work here to help women and to bring new life into the world, so for three women to die in a unit where this is almost unheard of has been a shock for them."

The hospital has the second-busiest maternity unit in Greater Manchester and last year helped deliver 4,324 babies.