But she admitted that her family’s situation was unusual in some ways, saying that she had suffered “pretty bad” morning sickness during the first three months while forming a new government, and that she did not know “how the government cars would feel about having a baby seat in them.”

Ms. Ardern said she planned to work right up until she gives birth and would then take six weeks of parental leave. During that time, she said, the deputy prime minister, Winston Peters, will assume her duties.

After that, Ms. Ardern added, she planned to return to “full duties,” with Mr. Gayford traveling with her and their child as often as possible.

Ms. Ardern will be the first leader of New Zealand to take parental leave. Two other women have also served as the country’s prime minister, Helen Clark of the Labour Party, who offered Ms. Ardern her congratulations on Friday, and Jenny Shipley of the National Party, who had children before she took office.

Mrs. Shipley offered words of support on Friday, saying Ms. Ardern would be able to juggle the jobs of prime minister and parent.