Hughes was found to have blood in her waters but the baby's heart rate was recorded as normal, between 120 beats per minute and 160bpm, at about 12.45am.
Hughes, 40, was noted as having high blood pressure and didn't want the fetal heart rate monitoring belt across her stomach any longer, probably because she was in such severe abdominal pain, according to the notes.
"... the fetal heart rate was finally picked up again on handheld doppler and at low rate of 67bpm at 1.19am," the notes state.
An emergency bell was rung and a duty doctor found Hughes had blood clots indicating a likely uterine rupture.
Her delivery priority was escalated to category 1 C-section which was immediately performed under general anaesthesia.
Hughes' partner, Martin Walters, the father of her 17-month-old twins born in 2017 via Caesarean, was not allowed in the theatre where she would lose four litres of blood.
Walters said he waited hours for news of baby Hohepa Hemara Walters-Hughes' birth before he was met by a doctor about 5am.
"I was just wondering why he took a donk to the head, is my concern. That's what the doctor said to me."
Photographs of the back of the baby's head, seen by the Herald, show large indentations Walters believes are not explained by the uterine rupture.
"I've got four other kids to another woman who had C-sections too. That's how I know that babies don't come out like that. I've had enough to know that every C-section, my baby's head don't come out like that."
The notes state that baby Hohepa and the placenta had been "expelled from the uterus to the abdominal cavity" when Hughes' uterus ruptured.
"The baby was delivered feet first, in a poor condition at 1.22am."
Hohepa, who weighed a healthy 3450 grams [7.6 pounds], was rushed to the Newborn Intensive Care Unit and placed on a cooling mat used to reduce brain damage in infants deprived of oxygen at birth.
However his injuries were not survivable and Walters and Hughes switched off the baby's life support that day.
"It was pretty sad because we had to take all the cords out and he had to breathe on his own and he was trying to cough.
"We were holding him while he was taking his last breaths. I never lost anybody like that."
No autopsy was performed after Walters, 33 and Hughes declined based on cultural beliefs.
Their baby was buried at Te Hape Marae in Benneydale, south of Te Kuiti, and his baby gear packed up or given away.
Walters said the couple were struggling to come to terms with their son's death and his siblings did not understand why their baby brother had not come home.
Waikato District Health Board quality and patient safety clinical director, Dr Douglas Stephenson, said the DHB was investigating the death.
"We can confirm that we are investigating the circumstances surrounding the death of a baby at Waikato Hospital on Friday, April 5, and have started a serious event review.
"We would like to express our deepest sympathy to the family at this very sad time.
"We have met with the family and will continue to work with them to address any concerns and keep them up to date as we review what happened."
He said Waikato DHB was not prepared to publicly discuss individual patient cases.
Uterine ruptures are rare. Rates during vaginal birth after one previous Caesarean are estimated at 0.3 per cent, or 3 in 1000.
Risk factors include vaginal birth after Caesarean (VBAC), other uterine scars, induction, obstructed labour and trauma.
The risk of death of a baby is put at 6 per cent and it can also be fatal for the mother in extreme cases.
