Neil Patrick Harris once shared why he prefers not to know which twin is biologically his, after leaving fans stunned with a striking detail in a recent photo.

Last week, the 52-year-old How I Met Your Mother star posted on Instagram to update his 8.9 million followers about the latest happenings with his and husband David Burtka's children, Gideon and Harper, who are now 14 years old.

The twins, who are fraternal as they originated from two separate eggs, were born via surrogate in 2010.

Both Harris and Burtka, 50, who is also an actor, contributed sperm with the hope that one would fertilize an egg. To their surprise, both eggs were fertilized, leaving the biological parentage of each child unknown.

Neil Patrick Harris shared a new snap of his teenage twins to social media last week (Instagram/@nph)

Harris and his husband David Burtka (Instagram/@nph)

"The kids are entering high school this year (how wait what??), and they are finally allowed to shed their Chromebook anchors and move up to the world of the MacBook. Huzzah!" Harris captioned the charming photo, leading many to marvel at just how fast the two have grown.

One fan commented: "Hey, I’m certain they were just toddlers a few weeks ago, so that can’t be right!"

"My knees just cracked—what do you mean they’re in high school!" joked another, while a third added: "When did they stop being toddlers?"

Harris and his husband, David Burtka, welcomed the twins via surrogate back in 2010 (Instagram/@nph)

And another echoed: "I was just starting high school when they were born! How time flies!"

Some claimed to have figured out which twin was biologically Harris', with one writing: "I always thought Gideon would resemble you, but wow. Beautiful preteen adults."

However, others disagreed, with some suggesting Harper looked more like Harris, while one fan remarked: "How is it that they truly resemble both of you?! They're stunning."

In a 2014 interview with American broadcaster and journalist Barbara Walters, Harris elaborated on why he has 'no interest' in discovering whether Harper or Gideon is biologically his.

He stated: "We used one of my sperm and one of David's to fertilize two eggs, hoping both would take, because we both wanted to be biological dads. Both took, miraculously."

Harris previously explained why he has 'no interest' in finding out which twin is biologically his (Instagram/@nph)

Harris also reiterated that he has no desire to determine which twin is his biological offspring.

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

While the couple remains unaware of the biological father of each child, Harris mentioned that they have some 'suspicions based on their behavior'.

However, the American Horror Story star did not disclose what those suspicions entailed, nor which child he thought might be his biologically.