Jacinda Ardern and Benazir Bhutto: A tale of two Prime Ministers

Jacinda Ardern might be New Zealand's first woman Prime Minister to give birth while in power - but she won't be the first in the world.

In 1990, Pakistan Prime Minister Benazir Bhutto gave birth to her second child, a girl. This apparently made her the first head of government in modern history to give birth while in office.

Pakistani opposition leaders criticised Ms Bhutto's pregnancy, saying she should not have had a child while in office.

"It is expected of those who are the leaders to make sacrifices," opposition leader Syeda Abida Hussain told United Press International at the time.

"But it is clear that the prime minister of Pakistan wants it all - motherhood, domesticity, glamour, and whole responsibility. She is even seeking more power than she has. Such a person in an ordinary context would be described as greedy."

Fearing her opposition would use her time giving birth to replace her, Ms Bhutto travelled incognito to a hospital and swiftly underwent a Caesarean section. She then returned to her job before she could be overthrown.

"The next day I was back on the job, reading government papers and signing government files," she later wrote.

"Only later did I learn that I was the only head of government in recorded history actually to give birth while in office.

"It was a defining moment, especially for young women, proving that a woman could work and have a baby in the highest and most challenging leadership positions."

But despite her best efforts, Ms Bhutto would only last another eight months before being dismissed.

In comparison, Ms Ardern will be able to take six-weeks off, with deputy Prime Minister Winston Peters filling in.

She has also been congratulated by Opposition leader Bill English.

"Mary and I would like to extend our sincere congratulations to Prime Minister Jacinda Ardern and her partner Clarke Gayford on the news of the Prime Minister's pregnancy," the former Prime Minister says.

"It is an incredible privilege to be a parent. A new child will bring real joy to their lives."