Ardern, 37, said she will continue working as close as possible to her due date of June 17 and then hand over power to her deputy Winston Peters.
As acting prime minister Peters will look after the day-to-day running of government, although Ardern said she will be consulted on significant issues.
"I'll still be receiving cabinet papers so I imagine it will be dialogue both ways," she told reporters.
"I may from time to time call the acting prime minister and he may at times chose to call me."
Ardern, whose centre-left government won office late last year, will be the first New Zealand leader to give birth while in office, and only the second in the world.
Pakistan's former prime minister Benazir Bhutto had her second child while in office in 1990 but resumed work almost immediately.
Asked how it felt to be the world's first elected leader to take maternity leave, Ardern responded: "Ask me after I give birth."
She said the power-sharing arrangement with Peters was the same as that currently used when she is out of the country on official duties, "only slightly longer".
"Broadly speaking, the deputy prime minister will exercise the function and powers of the prime minister in consultation with me if appropriate," she said.
On the campaign trail, Ardern had pushed back against questions about whether she intended to start a family, saying pregnancy should not affect a woman's career opportunities.
After the announcement that she was expecting a child, she said it would not impact her ability to do her job, telling reporters: "I'm pregnant, not incapacitated."
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