A newly released report on the deaths of babies is recommending changes to fertility treatments.
The Perinatal and Maternal Mortality Review Committee says the death rate for babies born in multiple births has increased from 32 per thousand in 2007 to 53 in 2011.
The committee doesn't know know for sure what has caused the rise, but suspicion has fallen on fertility treatments, including in-vitro fertilisation.
However, New Zealand's biggest fertility clinic, Fertility Associates, says rates of multiple pregnancies from IVF are declining.
It says a more likely culprit is a fertility drug called clomiphene, which can be prescribed by any doctor without monitoring.
The College of Obstetricians and Gynaecologists says another cause could be women having children later in life when they are more likely to have twins.
Meanwhile, the Waikato District Health Board has a higher rate of babies dying from a lack of oxygen in the brain than other DHBs.
The review committee wants DHBs nationwide to improve the standard of newborn resuscitation.
A report from the committee shows 149 babies were born with neonatal encephalopathy, a condition usually caused by a lack of oxygen in the brain at the time of birth between 2010 and 2011. Almost 40 of the babies died in the first 28 days.
Review committee chair Cynthia Farquhar said inadequate resuscitation contributed to the baby's illness in 16% of the cases.
Waikato has a significantly higher mortality rate from neonatal encephalopathy, with 2.25 deaths per thousand births compared to 1.27 for the rest of the country. The DHB was not available for comment on Wednesday.