"None of us in the cabinet virtually knew that this prime minister was about to deliver a baby", Javed Jabbar, a member of her cabinet, told the BBC recently.
"Ardern's pregnancy came as a surprise to her and Gayford", Ms Clark writes, "but they took it in their stride". Ms Ardern later broke news of the birth on social media, saying she felt very lucky and thanking staff at the hospital.
Prime Minister Malcolm Turnbull congratulated the new mother in a comment on her Instagram post.
In the interim, Ardern has passed her duties on to Deputy Prime Minister Winston Peters.
Ms Ardern shared a photo of her and partner Clarke Gayford with their daughter on Instagram on Thursday afternoon.
Ms Ardern is one of the few elected leaders to hold office while pregnant.
Twitter user @zaichishka, who describes herself as a Welsh-Kiwi, wrote: "4.693 million New Zealanders (give or take) just became aunts and uncles".
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Ardern's pregnancy has been followed around the world, with many hoping the 37-year-old will become a role model for combining motherhood with political leadership.
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Realistically, Ms Ardern will be hoping for a smooth six weeks while she's away, and Mr Peters will not want to be remembered for a vehicle crash of a time.
Her return to work will not be as swift as French Justice Minister Rachida Dati who went back to work only five days after giving birth in 2009.
She has said that she will still be on major decisions, such as issues of national security, and will keep reading briefings while she is off, but will leave the day-to-day running of government to Peters.
But for the public in New Zealand and beyond, the new arrival in our country's first family also has considerable significance.
'I do not want to create a false impression that all women should be super-human or super-women, ' she said. "I'll be Prime Minister AND a mum, and Clarke will be "first man of fishing" and stay at home dad".