Neil Patrick Harris admitted he doesn’t want to know which of his twins is biologically his

When actor Neil Patrick Harris and husband David Burtka became parents in 2010, the couple made a bold choice - to forgo finding out the biological father for each of their twins.

The pair's children Gideon and Harper - who are 13 years old - were conceived via surrogate, with the couple having previously opened up about what the process looked like.

NPH, known for his multiple roles in major US TV shows such as the classic How I Met Your Mother and Disney’s doctor OG Doogie Howser, M.D., told American broadcaster and journalist Barbara Walters in 2014 that he still had 'no interest' in finding out which twin is his biological child.

He has no interest in finding out. (nph/Instagram)

He said to her: "I have no interest in [finding out]. We are their parents and I love them implicitly.”

Explaining the process of how the pair used a surrogate, he added: "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.

"Both took, miraculously.”

Essentially, as the children are fraternal twins and not identical, the conception of the twins involved two embryos - one for each twin - and a surrogate to carry them. But the egg donor remained anonymous throughout.

Neil Patrick Harris has shared his parenting journey online. (nph/Instagra)

Gideon and Harper are now thriving with their dads and continuing to live life up in New York.

Even though the couple doesn’t know who the biological father is for each of their child, Neil went on to tell Barbara that they suspected who was who's 'based on their behaviour'.

But he didn’t reveal what those suspicions were, nor who he thought his genetic child was.

Neil and his family made the headlines last year when they shared a glimpse at their retro-style holiday cards, in a partnership with Minted.

The card featured the actor and his husband along with Gideon and Harper and their pet French Bulldog named Chunk, as well as 'an additional shot of the family dogs on the backside of the card'.

The family love to take photos of their trips. (nph/Instagram)

Fans quickly made their way to the comments talk about the card. One wrote: "Such a pretty card," while a second said: "Retro is where it’s at this season!"

A third added: "You two are so creative! I look forward to seeing what you all come up with next!"

- Neil also posted a tribute to the twins on Instagram on their birthdays in October last year.

The caption read: "And, just like that, they're teenagers,"

“Happy birthday, Harper and Gideon.

“You are the lights of my life. I can’t wait to watch you want nothing to do with me, and I promise I’ll be with you through all of it.”