Finally tonight, the medical miracle. The little boy behind me transforming two families. Here's erielle reshef. Reporter: That spellbinding sound. That is probably the coolest thing. Reporter: Nikita Nash, hearing her baby's heartbeat once again. I don't even know what to say. Reporter: Her son Lucas passed away when he was just 6 months old. The worst possible moment any person or any mother can imagine. Reporter: Four years after experiencing her deepest pain, this week, Nikita was finally ready to fly from Minnesota to Oklahoma for this precious moment. That is an indescribable feeling. It sounded like it did when I was pregnant with Lucas. Reporter: Little Lucas breathing new life into another baby from Tulsa. He was in cardiac arrest for 25 minutes. Reporter: Kolton Carter was diagnosed with several complex heart defects, receiving a heart transplant at three months. This letter of gratitude written to Nikita from Kolton's mom Jenny. In the dork -- darkest time of your life, you made so many brighter. From the bottom of your sweet angel's heart -- -- That beats inside my son's chest, we thank you. Reporter: Kolton, now 4. His health still an uphill battle. But his heart, thriving. Oh, thank you. Reporter: The two boys sharing a bond. Their two families forever connected. Lucas' heart is still getting to live on. I'm not just caring for Kolton. I'm caring for Lucas too. Reporter: Erielle reshef, ABC news, New York. A big thanks to those two moms for sharing their story. And thank you for watching. I'm Tom llamas in New York. Have a great night. Ribeiro: Here's a sneak peek at tonight's