On Thursday morning, Canadian Prime Minister Justin Trudeau announced he and his wife Sophie Grégoire are separating after 18 years together.

The couple shared the news in a statement on Instagram, saying how they made the decision after "many meaningful and difficult conversations".

Since their wedding in 2005, Trudeau and Grégoire have remained fixtures in Canadian public life. The couple have three children together.

Long before their relationship though, Grégoire was childhood friends with Trudeau's late brother, Michel.

Watch: Justin Trudeau on feminism and fatherhood. Post continues below.

"We could play Tarzan for hours," she told Maclean's about her friendship with the youngest Trudeau brother.

In 1998, Michel was killed in an avalanche in British Columbia.

He was only 23 and his body was never recovered.

The Trudeau family eventually called off the recovery search and built a chalet nearby as a memorial for him.

The Canadian Prime Minister has written about the moment he was told his brother had died.

"Michel had been doing what he loved most when he died, backcountry skiing with friends. I felt a spasm of guilt. What was Michel doing out on that glacier? Why hadn't I, as his older brother, found some way to protect him?" he wrote in his 2014 memoir.

"I should have visited him more, called him more, watched over him more, done something to keep him from danger."

Five years after Michel's death, Trudeau and Grégoire crossed paths again. They were at a charity event, and started chatting and flirting.

At the time, Grégoire was working as a TV reporter, covering entertainment.

"I had found my calling," she later said, reflecting on the moment she found a career in radio and television.

After their first meeting, Grégoire sent Trudeau an email, but he never replied to her.

"I knew if I responded even slightly, we'd wind up going for coffee, and that would be the last date I'd ever have in my life," he told Vogue in 2015.

A few months later, the pair bumped into each other on the street. Trudeau apologised for not responding to her email and asked her out to dinner.

Grégoire said it took her a while to accept the dinner invitation, but she eventually said yes.

They went to a quiet restaurant, then onto a "dingy karaoke bar" to sing Elton John and The Beatles.

"I'm a dreamer and a romantic, and at the end of dinner, he said, 'I'm 31 years old, and I've been waiting for you for 31 years.' And we both cried like babies," she said of the night.

In 2005, the couple married.

After the wedding, Trudeau ventured into politics - and that, of course, impacted Grégoire too.

She transitioned from her original career in media to becoming a yoga instructor and public speaker in the health and nutrition space.

Trudeau had previously said in an interview that he wanted "at least three" children, though he and Grégoire didn't see eye to eye.

"Sophie was an only child, so she says one is just fine, two is good, too, but three, you're starting to push it a little bit," he said.

"But I'm very confident that once we get into it, I'll be able to convince her that three is fine."

And that he did.

The couple welcomed their first son, Xavier, in 2007, their daughter, Ella-Grace, in 2009, and their third child, a son named Hadrien in 2014.

The same year their youngest child was born, their family home was broken into. Trudeau was away for work and Grégoire was home alone with the children.

The man, then 19 years old, had no charges made against him as it was found he had come into the home by accident, trying to locate his friend's nearby home.

He was highly intoxicated when the break-in occurred, and thought about stealing a block of knives from the kitchen, his lawyer said, but decided against it.

Instead, he left a note saying he could have stolen them, and that the family should lock their doors.

It left an indelible mark on Grégoire, and their approach to personal security changed as a result.

"We're extremely shocked and destabilised," Trudeau said following the break-in.

"It's a reminder of the need to be vigilant. We have an incredible country here where I wander around with no need for security and that's wonderful, but we do have to be very, very careful about how we move forward."

Around the same time, Trudeau and Grégoire were also in the news following allegations the PM had been unfaithful.

During a 2014 interview with theCanadian Broadcasting Corporation, he was asked directly about the rumours.

"Our marriage isn't perfect, and we have had difficult ups and downs, yet Sophie remains my best friend, my partner, my love," Trudeau said.

"We are honest with each other, even when it hurts."

He clarified that no extramarital affair had taken place.