Prime Minister Jacinda Ardern has opened up about the lengths she and partner Clarke Gayford went to to keep her pregnancy under wraps.

In an interview with The Times, Ardern revealed she had secret after-hour appointments with her GP before announcing she was pregnant with her first child in January.

"Even trying to organise secret scans was quite odd. My GP opened up after hours for me. She knew an obstetrician who could see us after hours and I would pop in at nine at night. He would make sure that the back alley was clear of the guy from the chicken shop who used to smoke outside. It was all very New Zealand," she said.

Ardern, 37, said her GP and Gayford, 40, were the only people who knew that she was pregnant the day she was sworn in last year.

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It was around that same time that her morning sickness started.

"It was awful. But we were at the beginning of our Government — I had to tolerate it," she said.

"My mother had morning sickness for the entire nine months so even though I felt rubbish I thought '70 per cent of women are at some stage in this situation so get on with it and stop feeling sorry for yourself'."

However Ardern admitted: "When I got morning sickness I said to my partner, this is it: Only one."

Ardern said her pregnancy was a complete surprise to the couple, who had sought assistance in the past to have a baby, but it didn't work out.

Gayford was filming an episode of his TV show, Fish of the Day, in the far north when she told him the big news.

"Two years ago I was living in central Auckland with none of this. In the space of a year I had a by-election, a new seat, deputy leader, leader and then prime minister and pregnancy — and I lost my grandmother and my cat, it was a bit of a whirlwind."

And that whirlwind was no doubt set to continue with Ardern planning to take just six weeks' maternity leave before returning to her role as Prime Minister.

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"My grandmother died during the campaign. She would have found it difficult, me working and having a baby. I grew up in a rural environment. One side of my family was reasonably conservative. My grandmother might have liked me to be married, though she got over it," she said.

"I can create space in my office [for breastfeeding and nappy changing], my partner can help a lot, so can my family. There are lots of things that will make it easier for me than other women."

Ardern said Gayford would not totally give up his job to be a dad and that the baby might go on some fishing trips but that his work was "being significantly curtailed".

But Ardern said her situation was no proof women could have it all and that there were sacrifices in every choice.

"I don't want to create the impossible idea that women can do absolutely everything."

The Prime Minister also told The Times she was debating what to do with her placenta and the umbilical cord when she gives birth in June.

Last weekend, Labour MP Peeni Henare suggested Ardern bury her placenta at Waitangi according to Maori custom.