Junior midwives taking riskier cases is responsible for a higher rate of baby deaths compared to their colleagues with up to five years experience, the ministry of Health says.
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It follows the release of a study showing inexperienced midwives are taking on women with high risk factors including ethnicity, hypertension, obesity and diabetes, without adequate support.
Ministry of Health's Stewart Jessamine said people tended to think all pregancies and the associated risks were the same, but that wasn't necessarily the case.
"There are a number of things that expose you to different risk. In New Zealand women under 20, this being the first pregnancy. The first pregnancy is always more dangerous that every other pregnancy, up to about pregnancy five or six."
Smokers, and women who lived in areas of social economic deprivation, were also exposed to risk.
Asked why junior midwives ended up with these high risk cases, Dr Jessamine said that wasn't entirely clear.
"Women who fit these higher risk criteria often present late, later in pregnancy, and if you are an established midwife you have a caseload of women you've previously delivered in the past."
The study also found first year midwives have a higher rate of baby deaths, compared to their colleagues who have been in the job for up to five years.
But there was no difference between junior midwives and those with more than five years experience.
It also found midwives who delivered fewer than 30 babies a year, had a significantly higher risk of perinatal mortality than those with an annual caseload of between 50 and 80 births.
The study centred on the care provided by community lead maternity care midwives who in 2014 accounted for 85 percent of the country's births.
The Health Ministry ordered the study following the release of a controversial study by the University of Otago in 2015, which found babies were more likely to die at birth or soon after if the midwife was in her first year in the job.
That study matched midwives level of experience to baby mortality between 2005 and 2009.
It found a higher likelihood of a baby dying at or near birth if the midwife - with midwifery only training - was in their first year in the workforce compared to midwives with between five and nine years training.
The latest study - which is 16 months late - found midwives with less than a year's experience were associated with 148 perinatal deaths during the seven year study period - at least 30 of these were due to inexperience, compared to their more senior colleagues.
Read the full study report
It also found first year midwives had a significantly higher perinatal mortality rate, even when risk factors were taken into account, compared to women under the care of a midwife with between two to five years experience.
"There was a significantly lower perinatal mortality, after adjusting for known potential confounders, among women under the care of second to fifth year midwives, compared to first year of practice midwives," the report said.
"In other words, if second to fifth year of experience are set as the referent group, then first year of practice midwives have increased odds of perinatal mortality," it said.
Dr Jessamine said both studies were interesting.
"The first one asked the question, 'If you were cared for by a first year midwife, as opposed to a midwife with 5-10 years experience, was there a difference in the neonatal deaths?' and the [answer] was 'yes, it was'. But that's all it looked at, it didn't look at, was there a reason for that, other than to do with the fact the midwife was first year or years 5-10?"
The second study looked at a different data set and time period, and not the same group of women.
"It looks at all the birth for about six years, it looks at all of the cases that were managed by midwives and identified that if you just do the first year against the fifth year, there is a difference in mortality. But if you look at why, what comes out is that the first year midwife case mix has more complicated cases in it, than midwives who've been in practice for five years and if you adjust for that there's no difference between the two," he said.
Useful study, but it doesn't provide reassurance
The lead author of the 2015 study, Professor Beverley Lawton, has welcomed the findings saying both study's found a statistically significant reduction in the odds of perinatal mortality among women under the care of midwives beyond the first year of practice, compared to those in their first year of practice.
"What the paper quite clearly shows is that without controlling for these clinical risk factors in a woman there was an increased perinatal mortality for first year midwives. When they controlled for their risk there was still an increased rate of perinatal mortality for first year midwives, but this does not reach the level of what they call statistical significance."
But Victoria University's Professor Lawton said she didn't think the public could be reassured by that, because in the process of the undertaking the study around 53,000 births (out of the 344,910 births included in the study) had to be removed because of lack of information.
She said removing that number of births reduced the study's "statistical power".
"I don't think we can be reassured by that because in the process of doing this they also removed about 53,000 births because they had incomplete data and that may also affect that result," she said.
Just because it didn't reach statistical significance doesn't mean it's not a real effect, she said.
No plans to change midwife training: Minister
Meanwhile, the Health Minister David Clark said there were no plans to change midwifery training in the wake of the study's findings.
Mr Clark said the Ministry of Health was meeting with midwives over pay and conditions, but there are no plans to ensure there's more training.
"I have no present proposals before me in terms of concrete changes to training, but I do want to ensure that the model is right and midwives are appropriately supported to do their work in the community," he said.
Mr Clark says he had put in a budget bid to address the funding disparity between DHB and community midwives, who are paid much less than their hospital counterparts.