This makes Ms Ardern only the second sitting prime minister to give birth in office, following from former Pakistani Prime Minister Benazir Bhutto who had a daughter, Bakhtawar in 1990.
The baby arrived at 4.45pm - shortly before 6am United Kingdom time - and weighed 3.1kgs, she said. In that short time, she took her party out of a near death zone of low polling to a highly credible election result, and then through the successful coalition negotiations which resulted in her becoming prime minister in October at the age of 37.
"Ardern is one of the few elected leaders to hold office while pregnant".
Jacinda Ardern's baby announcement on Instagram. "We already talk about significant issues, that will just continue, we'll just be in different roles", she said in an interview with Fairfax media just before going on maternity leave.
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A few moments later he finally chose to leave, kicking a ball away with his left foot before limping away beside the team doctor. For Neymar or any other player for Brazil , the pressure is always there, this is expected.
Arden had announced in January, three months after taking office as the world's youngest female prime minister, that she was expecting a baby with her partner, television fishing show presenter Clarke Gayford.
She added that both mother and daughter were doing well.
Ardern went to hospital early on Thursday, announcing she was handing over the top job to Deputy Prime Minister Winston Peters.
She's a choice little new mate.
Ms Ardern arrived at hospital with her partner on Thursday.
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Ardern has fielded several questions about whether she wanted children, but has told media outlets that she should not have to respond to such an inquiry.
Her duties as head of government will be handled by the country's Deputy Prime Minister Winston Peters, although she said she would remain in charge and read cabinet papers during her time away.
Well wishes have also come from former prime minister Helen Clark, Britain's Prime Minister Theresa May and US Ambassador to New Zealand Scott Brown.
The birth is being marked as a sign of progress for women.
She said that after the birth, she hoped to have some quiet time to enjoy as a family.
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The police have stated there is no danger to the public, denying that the shooting was a terror attack of some sort. Witness were quoted as saying that at least one attacker was armed with an automatic weapon.
The baby's name is still unknown - the couple recently admitted they were struggling with choosing a name.