Over his 40-year career, Dr. Carey Andrew-Jaja has delivered 8,000 babies and received the special nickname of "The Singing Doctor." That's because he sang to every single one of the babies he delivered — and then some.

Dr. Andrew-Jaja, who worked at UPMC Magee-Womens Hospital in Pittsburgh, loves to sing "Happy Birthday" and Louis Armstrong's hit, "What A Wonderful World," to the newborns.

He explained his song choice in a video for UPMC. "To me, it's a wonderful thing in my hand, the miracle of life," he said. "And the rest of is that it's a beautiful world we live in. We forget about all the crisis going on everywhere for a moment when you see that miracle of life in front of you."

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The OB/GYN said he took up singing to the babies when he was just starting his career. "I like to sing. But this particular part of singing to babies started when I was a resident," Dr. Andrew-Jaja said in the video.

Dr. Jaja's song of choice? "What A Wonderful World" by Louis Armstrong. UPMC

"There was a gentleman, an older obstetrician-gynecologist on staff and he loved to sing. And later on when he was about to retire, he asked me, 'Andy, do you sing to your babies?' And I said, 'Well, no. It's your stuff,'" Dr. Andrew-Jaja said.

"And he said, 'Go ahead, do it.' So, I took it over. He passed on the baton to me," he continued. "And I took it over and I started to sing to my babies ever since then and I do it every time."

Now, four decades later, Dr. Andrew-Jaja has sung his last lullaby at the hospital. At the end of June he officially retired, UPMC told CBS News.

He actually stopped delivering babies a few years ago, but kept seeing OB patients, UPMC said. It isn't uncommon for doctors to do this and although he wasn't at their deliveries, Dr. Andrew-Jaja would visit patients in the postpartum wing after their babies were born — and he would still sing.

He'd check on the moms and babies and give them the gift of song. Lindsay Grimes was one of the last moms to receive that gift.

"We're so excited to have a new addition to our family," Grimes told UPMC ahead of Dr. Andrew-Jaja's visit. "So, to have him sing to one of our babies would be awesome."

Baby Luella was one of the very last babies "The Signing Doctor" Dr. Carey Andrew-Jaja sang his lullaby to after 40 years. UPMC

The doctor visited Grimes and her newborn, Luella, and sung his signature: "What a Wonderful World."

Dr. Andrew-Jaya said that the last baby he sang to was just as important as his first. "Each of them is an individual. You know, I've delivered thousands and thousands of babies," he said in the UPMC video. "When I'm singing to those babies, I think I'm singing to a future important person. That's the credit I give to all of them."

While he's officially retired from both delivering babies and visiting patients, Dr. Andrew-Jaja's legacy as "The Singing Doctor" will continue in Pittsburgh. Councilman Corey O'Connor declared May 16 "Dr. Carey Andrew-Jaja Day" this year.