Miss Nathan's midwife, who was granted permanent name suppression, was relatively inexperienced.
In his findings, the coroner described the midwife's "successive errors of clinical judgment and failures to follow midwifery practice".
The coroner heard there were several warning signs that should have warranted a referral to an obstetric consultation, which were ignored by the midwife.
Whanau spokeswoman Jenn Hooper, representing the Nathan family and the family of the baby's father Hayden Tukiri, said the coroner's findings were "invaluable" in helping them understand exactly what happened to their two loved ones.
"Although it hurt, unbelievably so, to know that these deaths were preventable, the next step really is holding on to hope that these incredibly strong recommendations will now actually be made into reality."
Miss Nathan experienced a prolonged early stage of labour, and once labour progressed she collapsed in the birthing pool and fainted.
She roused and delivered baby Kymani. Both were rushed to Waikato Hospital. Kymani was in a "seriously compromised state" and died two days after his birth.
Miss Nathan died soon after arriving at Waikato Hospital, after suffering serious complications in childbirth and significant blood loss.
Mr Evans said Miss Nathan had suffered a rare medical emergency called amniotic fluid embolism, which caused widespread organ failure and post-partum haemorrhage.
He said Kymani had been deprived of oxygen when Miss Nathan fainted, as a result of the amniotic fluid embolism.
In the later stages of Miss Nathan's pregnancy the midwife noted that her fontal height, or size of her baby bump, was in the 90th percentile.
A referral to an obstetric consultation or an ultrasound could have shown the baby had oesophageal altresia, meaning the oesophagus ended in a pouch, causing a condition that meant Miss Nathan had more amniotic fluid than normal.
Specialist care in a hospital could possibly have meant a better outcome for both mother and baby, the coroner said.
The recommendations Mr Evans laid out in writing for the Minister of Health, Jonathan Coleman, the Ministry of Health and the Midwifery Council included an extended period of supervision and mentoring for graduate midwives, or for an extension on the teaching period for undergraduates.
He also recommended further guidelines be used around foetal growth charts.
Ms Hooper also acts as spokeswoman for Action to Improve Maternity and urged Mr Coleman to follow the recommendations that Mr Evans had made.
She said she was sceptical about whether the recommendations would be followed through by the Midwifery Council.
