Neil Patrick Harris and his husband David Burtka are the proud fathers of fraternal twins Gideon and Harper, who were born in October 2010.

The children, who were conceived via surrogacy, often make happy appearances on the How I Met Your Mother star's social media pages to mark milestone moments.

In a moving post celebrating their 14th birthday, Neil shared a sweet photo of them online. He penned: "My husband David and I marvel at their maturity, are wowed by their warmth, chuffed by their charm ... You're both amazeballs."

Fans also shared their best wishes in the comments, while others noticed the uncanny resemblance the twins shared with their parents.

Throughout the years, the Broadway star has opened up about the teenagers' conception and what he knows about their genetic makeup.

In a 2014 interview with Barbara Walters, Neil revealed that he had "no interest" in finding out which twin is his biological child.

The Doogie Howser, M.D star said: "I have no interest in [finding out], we are their parents and I love them implicitly." He also opened up about the intricate process he and David underwent to become parents.

The couple, who began dating in 2004, and got married 10 years later, used an anonymous egg donor and a surrogate to carry their twins.

Neil explained: "We inserted one of my sperm and one of David's sperm into two eggs, with the hope that they would both take, just because we both wanted to be dads biologically, and both took, miraculously."

As the children are fraternal twins, the conception of the twins involved two embryos - one for each twin. Neil then admitted that he and David had suspicions about whose child was whose "based on their behaviour,"

"It's so funny; I see myself in each of them at different times," he chuckled but he didn't reveal what those suspicions were, or who he thought his genetic child was.

During an appearance on The View, Neil opened up about addressing this to his children and the questions surrounding his and David's relationship.

He shared: "I think when our kids got to the age when they were having reasonable conversations where they're asking lots of 'why' questions, then it seemed [important] to be able to have a real clear-cut [identity] of who their daddy is - that he's my husband, it was easier than partner or boyfriend."