Malmstrom is a mother of two.
Famous faces born on June 21
If Ardern and Gayford's baby is born today it will share a birthday with royals, philosophers, human rights activists and musicians.
On this day, 36 years ago, Diana, Princess of Wales, gave birth to her first child, Prince William, at St Mary's Hospital in London.
A year later, in 1983, United States National Security Agency whistleblower Edward Snowden was born.
Also sharing the date of June 21 are Indonesian President Joko Widodo, United States musician Lana Del Rey and the late French philosopher Jean-Paul Sartre,
The news of Ardern going into labour this morning has sparked news coverage around the world.
The New York Times is leading the international coverage, posting a story under the headline "Jacinda Ardern, New Zealand's Prime Minister, is expected to have her baby soon".
CNN has joined the coverage too, noting Ardern is the "first pregnant world leader since 1990".
Over the ditch, Australian news outlets have also been quick onto the impending birth.
Sky News posted a "Just In" tweet this morning, revealing the news to their audience.
Kiwis are also reacting across social media this morning at the news.
#BREAKING
👶👶👶👶👶
Prime Minister Jacinda Ardern has been admitted to hospital.
BABY TIME!
http://www.thehits.co.nz/news/prime-minister-jacinda-ardern-in-labour-with-her-first-child/
Posted by The Hits on Wednesday, 20 June 2018
Famous people born on June 21
Prince William, 1982
Joko Widodo, Indonesia president, 1961
Lana Del Rey, musician - June 21, 1985
Edward Snowden, NSA whistleblower, 1983
Jean-Paul Sartre, French philosopher, 1905
Benazir Bhutto, former Pakistan prime minister and first female leader of a Muslim nation (also the first head of government to give birth while in office), 1953
Shirin Ebadi, Iranian lawyer and human rights activist and first Muslim woman to receive the Nobel Peace Prize (2003), 1947