Babies born under midwife-led care have suffered more incidents of damage than those whose mothers were looked after by doctors, according to new research.
The University of Otago study has been criticised by the College of Midwives, which says obstetricians are often not available when needed.
The college also says the study shows the inequality between private and public care and is not a reflection on midwives.
The ''unexplained excess'' of adverse events in midwife-led care is the major finding of the five-year study of New Zealand's maternity system published in the journal Plos Medicine.
Major findings include a 55% lower risk of birth-related asphyxia and a 39% lower risk of neonatal encephalopathy for babies under medical, rather than midwife, care.
Babies under midwife care were more likely to have a low Apgar score (a measure of infant wellbeing).
There was a lower rate of stillbirth and new-born babies dying under medical care, but the results were statistically weak.
Overall, rates of adverse maternity events in New Zealand are low.
Researchers, based in the university's Wellington campus, said the findings were of concern and warranted further research.
The study compared outcomes for women cared for by medical practitioners (obstetrician or GP obstetrician), with those cared for by midwives.
Mothers were grouped by principal carer. If they were under a midwife, and transferred to an obstetrician during labour, it would still be counted as midwife care.
In the study, 91.5% of women had a midwife as lead carer, and 8.5% of women had a medical lead carer.
Those with a medical lead fell into two camps - they had paid to go private, or were considered high-risk and were entitled to medical care.
New Zealand's maternity system changed in 1990, and this had ''occurred rapidly'', the study says.
''There is a need to understand the reasons for the apparent excess of adverse outcomes in midwife-led deliveries in New Zealand.
''Despite a radical change in the way maternity care was delivered, there has never been a full and proper evaluation to ensure the maternity system in New Zealand is safe,'' the study says.
Co-author Ellie Wernham, a master of public health graduate and former midwife, said it was possible better initial assessments were needed to ensure women who needed an obstetrician were assigned one.
But the aim of the study was not to compare midwives and doctors, it was to compare different models of maternity care, she said.
''I'm fully supportive of midwife-led care and have seen first-hand the many benefits that midwife-led care can bring. The overall aim of this research is to improve the current maternity system. It's not to revert back to the system we had in the 1980s where it was medically led and associated with a bunch of other problems,'' Ms Wernham told the Otago Daily Times.
College of Midwives chief executive Karen Guilliland said the study showed private maternity care was better funded and resourced than the public system.
''Most of our maternity hospitals are understaffed and often struggle to provide immediate response when midwives request medical input. This means that often women in labour have to wait to see a specialist, causing unacceptable delays for them and their babies.
''None of our main maternity hospitals have an obstetric consultant on site after hours or weekends, which are when the majority of births occur.''
