"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," she said in a Facebook post.
"I'll be Prime Minister AND a mum, and Clarke will be 'first man of fishing' and stay-at-home dad."
Speaking to reporters outside her home on Friday morning, Ardern said she would be taking six weeks leave following the baby's birth.
"I am not the first woman to multi-task, I'm not the first woman to work and have a baby, I know these are special circumstances but there will be many women who will have done this well before I have," she said.
Ardern said she learned she was pregnant in October during negotiations to form a minority New Zealand government, "but as many couples do in the early stages, we kept it to ourselves," she said.