Older mothers, more high risk pregnancies and better baby survival rates are pushing hospitals' newborn intensive care units past their limits.
Documents released under the Official Information Act showed a number of the neonatal intensive care units [NICU] and special care baby units [SCBU] in New Zealand were regularly at or above capacity.
The four biggest units in the country, at Middlemore, Auckland, Wellington and Christchurch Hospitals were frequently above 80 per cent occupancy over the last five years.
Auckland exceeded 100 per cent twice in this period, Middlemore three times, Wellington 13 times (three in 2016) and Christchurch 11 times (once in 2016).
Of the remaining six DHBs, which responded to the Herald's request for information, all bar two had exceeded 100 per cent capacity - which at times led to pregnant mothers or babies who were stable being transferred.
MidCentral District Health Board operations director, Specialist Community & Regional Services Nicholas Glubb, said there were a number of contributing factors increasing pressure on units.
"Increased incidence of high risk pregnancies including obese and older mothers, drug and alcohol related issues, increased social deprivation, multiple births and mothers with-comorbidites."
He said there were also more "late-preterm deliveries" of babies between 34 - 36 gestation as a result of "increasing maternal age, increased body mass and other illnesses and increased monitoring of fetal wellbeing and avoidance of risk by obstetric teams".
"These babies stay several weeks and cause the unit to exceed capacity more often."
Wellington general manager hospital and healthcare services, Chris Lowry, said the nationwide network of neonatal units was well equipped to "support the fluctuating demand across the country".
Typically babies were treated in the DHB of the area in which they lived unless reasons, such as needing more intensive care than their local hospital could provide, dictated otherwise and moved back to their area once stable.
If a neonatal bed was not available, the DHBs indicated transfers tended to happen prior to a baby's birth, or only if a baby was stable enough to be moved.
Auckland District Health Board chief executive Ailsa Claire said the small increase in births and medical improvements that have increased survival rates "may well drive future demand for neonatal cots, so prudent management requires this type of regional planning".
The New Zealand Nurses Organisation professional nursing advisor Kate Weston said despite pressures across the board, New Zealand's neonatal units were "world class".
But she said it was important to have a discussion on how to alleviate these pressures.
She said a potential answer moved increasing bed spaces alone - but looking at issues from a wider public health perspective.
"Care is being provided and is being provided to a high standard, but if the numbers are increasing, what are we doing to prevent babies who are born premature and unwell?"
She said issues such as an ageing birthing population, burden of disease impacting women, and mental health and addiction issues were all linked with more babies being born needing neonatal support.
Weston said with continual improvements in best practice and technology also pushing the boundaries of viability led to more babies surviving - but also needing care for longer.
"New Zealand does have a really good reputation internationally and provides a really good level of care - we would want to continue with that."
Average monthly occupancy rates in 2012 to 2016
51 - 97 per cent
49.12 - 119.16 per cent
Capital & Coast District Health Board (Wellington)
72 - 113 per cent
Hawke's Bay District Health Board [SCBU]
59.14 - 124.19 per cent
Taranaki District Health Board
66 - 143.3 per cent
Mid Central Health
46 per cent - 115 per cent
Waitemata District Health Board
65 - 98 per cent
80.3 - 100.5 per cent
Auckland District Health Board
75 - 102 per cent
Canterbury District Health Board
45 - 150 per cent