After that, Ms. Ardern added, she planned to return to "full duties" with Mr. Gayford and travel with her and her son as often as possible.
Mrs. Ardern will be the first New Zealand leader to take parental leave. Two other women have also served as the country's first minister, Helen Clark of the Labor Party, who offered her congratulations to Mrs. Ardern on Friday, and Jenny Shipley of the National Party, who had children before taking office.
Ms. Shipley offered words of support on Friday, saying that Mrs. Ardern could juggle the work of the prime minister and the parents.
"She will also be surrounded by smart people who will help her do her job, and I think it is another innovative example of what women can do," Ms. Shipley told Radio New Zealand.
Ms. Clark, a former mentor to Ms. Ardern who led the United Nations Development Program after being elected prime minister in 2008, said all women "should have the option of combining family and career."
Social networks were flooded with messages of celebration from New Zealanders after the announcement, with the silence of critics who questioned Mrs. Ardern's family plans before taking office.
Mark Richardson, the television host who started the debate last year by asking Mrs. Ardern if she planned to have children, has not commented. At that time, Mrs. Ardern, then leader of the Labor Party of the center-left, asked the journalists if they had asked the same question to a man.
Mrs. Ardern took office after a volatile election campaign, according to New Zealand standards. Former Labor leader Andrew Little resigned amid depressing voting numbers, pushing Ms. Ardern to what she, at that time, called "the worst job in politics."
But his candidacy reversed the fortune of the party, with his Labor Party winning 46 seats in the September 23 elections. The center-right National Party, led by Bill English, then the prime minister, won 56, not enough to capture the majority required to govern in the New Zealand Parliament.
The decision on who would govern was left to Mr. Peters, the leader of the minority populist party, New Zealand First, who maintained the balance of power.
After weeks of negotiations and deliberations, Mr. Peters supported Ms. Ardern, whose call for change and youthful energy strengthened Labor voters.
On Friday, Ms. Ardern said she had known of her pregnancy in mid-October, during the week that Mr. Peters announced her as her choice for the prime minister, but at that time she did not tell anyone in the negotiating table.
At the beginning of the tenure, Ms. Ardern helped approve a bill that extended parental leave paid in New Zealand to 22 weeks from 18, but her baby is expected to be born before the law takes effect 1st of July. New Zealand allows parents to split the assigned license between them, as Mrs. Ardern and Mr. Gayford plan to do.
It is believed that she is the only head of state in recent years to give birth while in office. In 1990, Benazir Bhutto gave birth to her second child when she was the first minister of Pakistan.
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