Differences in the medical and obstetric risks between the two groups don't fully explain this disparity, say the researchers, who looked at the impact of social, medical and obstetric factors on mode of delivery among women booked for privately or publicly funded care in the same hospital.

The researchers were particularly keen to see if funding source made any difference to operative deliveries, as it's not clear whether private care within a publicly funded setting prompts higher rates of costly interventions, they say.

Both private and public healthcare have been offered in Irish hospitals for decades at a ratio of 80 (public): 20 (private). And recent changes to UK health policy mean that hospitals in England will now be able to bump up the proportion of income they generate from private healthcare to 49%.

The researchers analyzed the method of delivery for just over 30,000 women with singleton pregnancies, in a large urban maternity hospital in Ireland between January 2008 and July 2011.

The hospital delivers between 8000 and 9000 women every year, with obstetric care provided by around 14 consultants and 16 trainees. Out of the total, 24,574 women were publicly funded and 5479 were private patients.

Private patients were more likely to be older, more affluent and better educated, and to be Irish than publicly funded patients. And they were less likely to be single, childless, have an unplanned pregnancy, or to have booked late for obstetric care.

And while they were less likely to have a medical disorder, they were more likely to have had fertility treatment, recurrent miscarriage, or experienced a previous stillbirth or infant death.

The analysis showed that compared with publicly funded patients, private patients were more likely to have a C-section or surgical vaginal delivery -- vacuum or forceps.

But the greatest disparity was in the rate of planned C-sections, which was around twice as high among the private patients, particularly for mums who had given birth before, and by C-section.

The differences remained after taking account of medical (including age), obstetric, and social differences between the two groups.

A common argument advanced for planned C-section is that the procedure does less damage to the pelvic floor than a vaginal birth, say the authors, but while private patients requested more C-sections, very few such requests were made.

"We found the differences observed in relation to operative deliveries were not explained by higher rates of medical or obstetric complications among private patients," write the authors, although older age and higher income may have played their part, they suggest.

But the findings raise important questions about equity, both in terms of use of resources and choice, and whether that choice really is in the best interests of the woman concerned, say the authors.

"Healthcare systems that include public and private patients need to reflect on the potential for disparate rates of intervention and the implications in terms of equity, resource use, and income generation," they comment.