But of course, women have to keep disproving it, again and again.
Witness, for instance, the unsurprising multitude of negative responses to The Post's story about Ardern, many of which were apparently posted by men. A slew of online comments attacked Ardern's marital status (she's not married to her partner, Clarke Gayford), her ability to keep her feelings under control (because lady hormones!), and the amount of time she's planning to take off to care for her newborn (a whopping six whole weeks).
Together, these remarks offer a depressingly thorough overview of the various misogynist stereotypes women continue to confront:
•"She should resign ASAP and focus on family. She is ineligible to govern . . ."
• "I guess millennial world leaders don't feel it's important to get married first."
•"That's just what New Zealand needs, a sleep deprived, emotionally volatile person running the show."
•"Women like this are responsible for breaking down traditional family values."
• "Shows irresponsibility as a prime minister and a parent, half a-ing both."
•"I just do not think women should be in such power if they are birthing babies. You can't argue with biology."
•"You have to choose between motherhood and leadership."
To be fair, there were many others who voiced admiration and joy for Ardern; a few brave souls even waded into the fray to point out that men are somehow exempt from similar inquisitions and expectations when they become fathers.
There's also the simple fact that pregnancy is a perfectly normal, healthy part of life for many women. It is not, for example, a serious illness, which can be far more distracting and debilitating, and which plenty of male leaders and lawmakers have found themselves facing while in office.
Consider the numerous United States presidents who have battled significant medical issues: Grover Cleveland, who had a secret surgery to remove a cancerous tumour from his mouth; John F. Kennedy, who took steroids to treat Addison's disease, an adrenal disorder; Dwight Eisenhower, who suffered a heart attack during his second term and later had a stroke. Four US presidents (William Henry Harrison, Zachary Taylor, Warren G. Harding and Franklin Delano Roosevelt) have died in office because of poor health. And plenty of male legislators - including, more recently, Maryland Governor Larry Hogan and Arizona Senator John McCain - have received serious or life-threatening diagnoses while in office.
None of these men resigned as a result of their conditions. And none of them faced an outpouring of public outrage because they dared continue serving as an elected leader while simultaneously having a personal life and a human body.
Ardern has already proven that her pregnancy isn't interfering in the slightest with her ability to govern; she pointed out that she managed to establish her entire government while juggling "pretty bad" first-trimester morning sickness. (Anyone who has experienced the misery of morning sickness will be the first to fully appreciate how impressive this is.)
When Ardern was asked how, exactly, she managed to push through her discomfort and do her job without anyone discovering her pregnancy, she answered simply: "It's what ladies do."
And, sadly, she's right. Guarding against attack by masking any sign of vulnerability; exhibiting the strength of will and body required to overcome a patriarchal double-standard; answering insulting questions about our fitness, despite overwhelming evidence of our qualifications - that is what ladies do.
One can only hope that maybe, with the help of Ardern's example, we might not have to do it for much longer.
Caitlin Gibson is a feature writer at The Washington Post
