Prime Minister Jacinda Ardern may have to wait up to a week for induced labour

Prime Minister Jacinda Ardern is entering the first day past her due date but may have to wait a week before labour is induced.

Ardern previously said she was keen to get induced if her baby did not arrive on time. And with only six-weeks of maternity leave planned, time is of the essence.

Auckland region obstetrician Dr Sue Belgrave said exactly when labour could be induced varied, but in most places around the world it was one to two weeks past the due date.

Induction involved starting labour artificially, rather than by itself.

"It is quite a common issue for women to face, with estimates from about 25 to 40 per cent around the country."

It was most common in situations when a woman's water had gone but they had not gone into labour, when the mother was overdue by a week or more, where a baby was a bit small, blood pressure was high or the mother had health complications such as diabetes.

Belgrave said induction was a medical, planned procedure.

"[Induction] involves using hormones, so very close to those released in the body when labour starts."

The timespan between induction and giving birth also varied.

"When women are being induced the timespan is a bit uncertain.

"Most will have their babies within a day, but some may take two to three days."

Ardern had previously joked she would try various methods to help induce labour naturally.

"I heard that curries, walking on kerbs, eating pineapple [help induce labour]," Ardern said

"I thought I'd do them all at once. So if you see me walking through Sandringham with a pineapple under my arm and a take-out curry … that's exactly what's going on."