Ardern and her partner Clarke Gayford chatted with the Queen in front of cameras, before the prime minister and Her Majesty spoke in private for about 20 minutes.
The meeting with the Queen is an "on appointment" audience, offered to new prime ministers from countries of which the monarch is head of state.
"I had a discussion with the Queen that mostly focused on New Zealand, she has a huge interest in how we're recovering from the many and varied things we've been up against in recent times and clearly, maintains great interest," she told reporters after the meeting.
She added that it was a "very warm" and "focused" meeting.
The PM also gave the Queen a photo of Her Majesty taken in the 1950s on a visit to Aotearoa and some New Zealand food items.
The Queen gave Ardern two photos of herself and Prince Philip for New Zealand.
Ardern did not go into detail about what was discussed or if the Queen commented on her pregnancy - as it would be against protocol to divulge exactly what they spoke about - but she was full of praise for the monarch.
"Here is a remarkable leader who has conducted her life in the full view of the public and that has included raising her children, and there's something to be admired in that."
Ardern said the visit with the Queen has not changed her view that New Zealand would one day become a republic.
"My particular views do not change the respect that I have for Her Majesty and for her family and for the work that they've done for New Zealand. I think you can hold both views, and I do."
The prime minister and Gayford later went to the Commonwealth Head of State banquet dinner at Buckingham House, where Jacinda Ardern was given the honour of proposing a toast to the Commonwealth and also got to speak with the Queen again.
Ardern wore a dress especially made for her by Kiwi designer Juliette Hogan and a traditional korowai, which was loaned to her by Ngati Ranana (London-based Maori).
At her opening address ahead of the CHOGM banquet, the Queen said it was her desire that her son Prince Charles, Prince of Wales, would succeed her as Head of the Commonwealth.
Ardern met with Prince Charles, and Camilla, Duchess of Cornwall, at Clarence House yesterday.
It was a big day for Ardern, who has also been named in Time Magazine's 100 most influential people list.
This is the prime minister's last overseas visit before she is due to give birth in June.