Jack was only three pounds at the time of birth but has now grown up to become a perfectly healthy and happy little boy.
"As many of you may know, our son Jack was just over 3 pounds when he was born. Thanks to scientific advances made possible by #marchofdimes funding, today he's a perfectly healthy, curious 4 year old," Pratt wrote captioning the video of his son playing with a Praying Mantis.
"Today is #worldprematurityday. 15 million babies are born #prematurely each year around the world. And 1 million of them won't live to celebrate their 1st #birthday.
"The @marchofdimes strives for a world where every #baby has a fair chance, yet this is not the reality for many mothers and babies. Join me in supporting their efforts to give every baby a fighting chance by clicking the link in my bio.