It’s been roughly three years since Reddit co-founder Alexis Ohanian found out he was going to be a father, but he’s still grappling with the profound awareness of his own mortality that came with it.

It first hit him when tennis star Serena Williams, his now wife, presented him with a bag full of positive pregnancy tests, and again a few months later when she almost died after giving birth to their daughter, Olympia. Now, he thinks it’s here to stay.

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“That overwhelming sense of mortality is the hardest, biggest thing that I suspect I’ll be spending the rest of my life really wrestling with,” Ohanian, 36, says. The multimillionaire venture capitalist is speaking from Palm Beach, Florida, where he is working from home, having just collected Olympia, now two, from “school”.

The US is one of only three countries in the world not to have any federally mandated parental leave, and the vast majority of American workplaces do not offer it.

But things were different for Ohanian, who was able to take 16 weeks’ paid paternity leave from his then employer Reddit (he now works full time for Initialized Capital, an early-stage venture capital fund which he also co-founded).

“I couldn’t imagine going through that as a husband and having to decide or even think for a second about choosing between your wife and your career. That is an inhumane choice to expect someone to make in a modern society.”

The experience had such a profound impact on Ohanian that he has since become a prominent voice in the US on fatherhood and an advocate for a national paid family leave law.

“My wife and I … we have means, we run our own careers, we have family, we have literally every advantage we could ask for when a newborn shows up, and yet still it was such a traumatising period,” he says.

Facebook Twitter Pinterest Serena Williams’s husband, Alexis Ohanian, holds their daughter, Alexis Olympia Ohanian Jr, at a tennis match in Perth, Australia. Photograph: Paul Kane/Getty Images

Williams, 38, narrowly escaped with her life when she suffered a pulmonary embolism after giving birth by emergency C-section. Following multiple operations, she was bed-bound for the first six weeks of Olympia’s life and many of the childcare duties fell to Ohanian. He had never held a baby before.

“I learnt how to do a diaper and I learnt how to do all these things and just get comfortable with this tiny little person that I was now responsible for.”

That crash course gave him the self-belief to handle future challenges. “I feel a confidence and a calm that is a direct result of having that time early when I had no fucking clue what was happening. But I figured it out and it worked.”

From the beginning he has shared his experiences of fatherhood on social media, which he believes is uniquely placed to normalise fathers as caregivers and undo negative stereotypes such as the “Homer Simpson incompetent dad”.

He believes a culture change is under way – evidence of which he claims can be found in his social media feed and group chat which he says is full of “dads doing dad things”.

“What social media has unlocked is more and more boring, everyday examples of dads in that role [of caretaker]. Just dadding, not babysitting, but just dadding, and that’s an important shift.”

As more high-profile US figures take time off for the birth of their children, such as Chance the Rapper and the baseball player Daniel Hudson, Ohanian says, there has been “a bigger sea change in perception around fatherhood, around masculinity”.

Ohanian and co-founder Steve Huffman started Reddit after graduating from the University of Virginia in 2005 and sold it the following year for a reported $10m to $20m. It’s now one of America’s most popular websites.

He says a big part of encouraging men who have access to paternity leave to take it is about redefining what male success looks like. “These things are not mutually exclusive. A man can be just as career-driven, just as passionate, just as effective whether or not he takes time off to be with his family when welcoming a new child.”

Facebook Twitter Pinterest Serena Williams, who was then expecting her first child, and Alexis Ohanian at the Metropolitan Museum of Art in May 2017. Photograph: Neilson Barnard/Getty Images

Ohanian says he did not consider the practicalities of paternity leave until he was about to become a father. When Reddit first introduced its policy he thought it was “reasonable” but admits he did not compute its significance.

“It wasn’t until a couple of years later when I was using the policy that I realised this is indispensable and it’s something that a lucky few get access to in America, that really everyone should.”

Last year, he went to Washington DC with other fathers to meet with members of Congress to talk about the issue as part of Dove Men+Care’s Pledge for Paternity Leave. He says they “came away from that trip feeling pretty optimistic because everyone there on both sides of the aisle agreed that this is something that we need”.

The biggest questions, he says, were around how to fund it, but that he is “glad we’re already at that part in the conversation”. He also takes hope from recently passed legislation entitling 2.1 million federal workers to 12 weeks of paid parental leave.

Ohanian will not single out which of the multiple proposed bills on family leave he supports, but in terms of paying for it, he thinks the paid family leave policies that have been introduced in some states – through an insurance fund model – could work nationally. While in an ideal world he would like to see new parents get six months off, he appreciates that may be ambitious.

It wasn’t until a couple of years later when I was using the policy that I realised this is indispensable Alexis Ohanian

Having lost his mother to brain cancer, he would ultimately like a family leave bill to include time for caring for sick relatives. “I knew what it was like in my early 20s to have to take care of a dying and really debilitated parent and luckily, again, I was an entrepreneur and so I could build the schedule that I wanted to build and I had that flexibility, but the reality is not many people have that.”

In the absence of a national law, private companies, especially in the tech industry, are increasingly offering paid family leave as an employee benefit. When Facebook’s CEO, Mark Zuckerberg, announced his leave after the birth of his second daughter, he took it as an opportunity to publicise the company’s four-month policy.

Ohanian, who excitedly refers to the competition between companies on parental leave as an “arms race”, predicts that in the next 10 years, as demand for tech talent rises across other industries, more companies will start offering it.

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It’s also valuable for employee retention and productivity. “My instinct here is that yes, an employee will be more productive if they have some sense of peace over how their castle is doing, how their home is doing.”

Initialized Capital offers 16 weeks’ paid parental leave. Last year, a quarter of their employees – including three fathers – took it. Ohanian, who refers to himself as a “business dad”, has also started investing in “family tech” companies like The Mom Project, a recruitment service for mothers, and the childcare app Kinside.

To get to gender equality, though, he says it’s essential that men take as much parental leave as women. “No matter who’s getting pregnant, if both parents are taking leave then it sort of nullifies that penalty for having a uterus.”