Prime Minister Jacinda Ardern is likely to miss out on her government's planned extension to paid parental leave.

Ardern posted on her Instagram account today that she and partner Clarke Gayford were expecting a baby in June.

"Clarke and I are really excited that in June our team will expand from two to three, and that we'll be joining the many parents out there who wear two hats.

"I'll be Prime Minister AND a mum, and Clarke will be 'first man of fishing' and stay at home dad," she posted.

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But the timing of the baby's arrival means Ardern or Gayford will likely miss out on the increase from 18 to 22 weeks - which doesn't kick in until July 1.

The extended scheme will then stretch further to 26 weeks from July 2020.

Ardern is not the first leader of a state to be pregnant while in office.

Former Pakistani Prime Minister Benazir Bhutto had her second child in 1990, becoming the first modern head of government to give birth while in office.