Plenty of women have multitasked before me: Ardern
The charismatic Ardern follows Pakistan's PM Benazir Bhutto in 1990 as the only women in recorded history to give birth while in power, but she downplayed the significance.
"Lots of people juggle a lot of things in their personal and private lives, and I'm not unusual in that. Plenty of women have multitasked before me," she said when revealing the pregnancy in January.
Ardern takes six weeks maternity leave
The birth capped an eventful-year for Ardern who became prime minister last October, three months after inheriting the leadership of the Labor Party.
When no party won sufficient support in the election, she was anointed prime minister by New Zealand's First leader Winston Peters.
Peters, who became Ardern's deputy, is now acting prime minister while she takes six weeks' maternity leave.
Opposition leader sends congratulations
New Zealand opposition leader Simon Bridges offered congratulations. "Being parents is a joy and a privilege and we are sure your new arrival will fill your lives with all the happiness our children have ours," he said.
Ardern faced sexism row on her becoming mother and PM
Ardern waited until January to reveal her pregnancy through a Facebook post.
Her plans for a family had sparked a sexism row, when a television host said voters had a right to know before they cast their ballots.
"It is a woman's decision about when they choose to have children and it should not hinder her opportunities in the workplace," she said then.
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Bhutto's daughter "first child born to a sitting female PM"
Pakistan's Bhutto, who was assassinated in 2007, gave birth in January 1990 to her daughter Bakhtawar who now describes herself on Twitter as "1st child born to a sitting female Prime Minister".