28% of births Caesarean: Report
Northern Ireland has the highest level of Caesarean section births in the UK or Ireland, it was revealed.
More than a quarter of pregnancies ended in the procedure, an audit office report said.
Each Caesarean section birth costs the health service almost twice that of a normal delivery.
Performing the operations in cases where they are not medically necessary can put mothers and babies at risk of infection and extend the length of stays in hospital, the review said.
Comptroller and auditor general Kieran Donnelly said: "In the current financial climate it seems particularly important that clinical managers in the health and social care trusts understand and manage the cost implications of different modes of childbirth.
"The use of a classification system such as that demonstrated in this report will be an essential aid in assisting health and social care trusts to verify that resource allocation is determined on the basis of clinical decisions."
In 2011/12, 28% of Northern Ireland's 25,703 births were by Caesarean section. That compares to 25% in England.
They typically cost the health service almost twice that of a normal delivery, £3,724 compared to £1,933.
Since the 1970s caesarean section rates have risen around the world. The global average stands at about 16% but there are enormous regional differences, even among western countries.
The report added: "While the reasons for the global increase are not entirely clear, it is likely that changing demographic characteristics such as rising levels of maternal obesity and increasing maternal age in childbirth have influenced rates of Caesarean section births."
Caesarean sections are the most common surgeries carried out in maternity care.
They can be a life-saving procedure for an infant in distress or where there are other labour complications, the report said.
The department of health's maternity strategy acknowledged that interventions save lives but said there may be scope for reducing variations across maternity units.
While the Mater Hospital in North Belfast has a caesarean delivery rate of about 23%, almost 36% of births use the procedure at Daisy Hill in Newry, Co Down.
The audit office said: "Although some variation would reasonably be expected, given differences in patient populations, the scale of the variation may be indicative of variations in clinical practice.
"The use of a classification system, such as the one demonstrated in the report, to produce data that increases clinicians' understanding of birthing interventions can be a major driving force in reducing variation."