Counties Manukau DHB is investigating after a woman lost her unborn baby after 20 weeks gestation.
The death is one of two potential "serious adverse events" in women's health services this year, the Herald on Sunday can reveal.
The DHB wouldn't release more information while inquiries continue, to protect the process and privacy of patients involved.
The incidents follow a damning internal review of maternity services last year, carried out after resourcing problems such as a lack of staff and bedspace at Middlemore Hospital contributed to the death or stillbirth of three babies over 2016 and 2017.
A serious adverse event is an incident where a patient is seriously harmed during medical treatment. There have been two potential cases in women's health services this year.
In April, an incident "resulted in" intrauterine death (a loss at or after the 20th week of gestation), the DHB confirmed. Another investigation was started in June, after a swab was mistakenly left inside a patient after surgery.
Incidents are considered potential serious adverse events while under investigation, because how severe an incident is considered can change.
Wider changes to maternity services have been made or are under way after the September 2018 review, which found an 11-bed shortfall and concluded maternity care for South Aucklanders fell below safe standards.
Counties initially declined to say if serious adverse events had happened since the September review. It has now disclosed the two incidents after an Official Information Act request.
Chief executive Fepulea'i Margie Apa declined to provide other details, including if there were any potential resourcing problems.
"Information on all completed serious incident investigations is published annually (in December) ... this occurs with the consent of the affected patients, and ensures that we protect the privacy of all individuals involved," Apa wrote in the information release.
"We do not believe the public interest in these matters outweighs the high privacy interests involved, particularly given the small number of cases, and our wish to maintain the ability for our staff to fairly participate in these investigation processes, and providing relevant information in confidence."
Apa said the DHB would be disappointed if the Herald on Sunday reported on the incidents, as "doing so would diminish the ability of the public to consider the full and proper context and understanding of these events".
The incidents needed to be viewed in the context of a total of 57 events in the year, across all DHB divisions. Counties' reported events were "reflective of our service size and efforts to encourage open disclosure".
Incidents where items like swabs are mistakenly left inside patients after surgery are relatively common - health boards reported more than 60 such incidents over the past few years.
Those involving the death of a baby or fetus are rarer. Two of 19 other DHBs reported a total of four such events in 2017/18, although a handful of health boards don't provide even summary information.
Auckland Mayor Phil Goff and local councillors including Efeso Collins and Daniel Newman have called for urgent help for Middlemore. Health Minister David Clark has indicated some support may be announced in the coming weeks.
The DHB's internal review last September - obtained by the Herald on Sunday along with other documents - outlines how Middlemore Hospital struggled to cope with big increases in the number of pregnancies needing more care and monitoring, and services buckled.
Other incidents include a new mother falling and snapping her shinbone after being discharged from hospital in February last year - six hours after giving birth and still numb from an epidural.
Births within the district have dropped to about 18 a day, but cases are increasingly complex, partly because of obesity and diabetes. More than a third of residents are among the country's most socioeconomically deprived.
After the review, the DHB launched one-year and three-year work programmes. Changes include a new maternity assessment clinic and more midwife, nursing and support staff roles.
There have been management changes, including a new general manager for women's health, as well as a full-time clinical quality and risk manager. A recently appointed chief midwife will be on the DHB's executive leadership team.
Women with babies in the neonatal unit are moving in phases to another ward, which allowed more beds to be opened in July and August. More capacity is anticipated by the middle of next month. A new women's health building is about five years away.
The Midwives' Union, Meras, is part of a regular forum at Counties Manukau DHB, and says positive changes have been made. Those include keeping more graduate midwives, more senior roles including in maternity wards, enabling clinical leadership 24-hours a day.