She lost weight and required multiple hospital admissions for severe morning sickness (hyperemesis), and her baby's growth was restricted, the Health and Disability Commission (HDC) report said.
However, her midwife did not record the woman's drop in weight or baby's growth at every antenatal assessment and specialist doctors who took over the care later on in the pregnancy weren't told.
The investigation found there was no formal management plan and no clear guidelines for staff to monitor the severe morning sickness and malnutrition.
When the baby was born, she was recognised as "at-risk" due to her low birth weight.
She was also administered a higher than recommended doses of anti-seizure medication.
Her condition deteriorated, and she was admitted to the Neonatal Intensive Care Unit, where she tragically passed away.
Deputy Commissioner Rose Wall said it was impossible to know whether the baby could have been saved but she was critical the DHB did not have adequate systems in place to support staff, including a requirement to develop comprehensive management plans in such complex cases.
"This extremely rare sequence of events for [the woman] and her whānau led to a tragic outcome for them with the loss of their baby," Wall said.
A family spokesperson said in the report that the whānau would like assurance that this will not be the case for any future pregnant mothers who find themselves in that position.
They strongly reject that the mum did not take her diet seriously.
"[She] tried to eat well but this was near impossible because of the persistent vomiting that developed during her pregnancy. "
"Her mental health through this period should have been taken into consideration, in particular the effects of being so unwell for such a long period of time," the spokesperson said in the report.
"The whānau would like to reiterate that it was a lack of appropriate resources and procedures that caused this
."
The deputy commissioner recommended the DHB update a number of its policies and consider the need to provide appropriate cultural support in complex cases.
They were also ordered to provide a written apology to the woman.
The midwife was also told to provide an update on the Order Concerning Competence issued to her by the Midwifery Council of New Zealand.
