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But, after years of fertility struggles followed by a miscarriage in early 2017, Mikey's mother, Jessica Marotta, says she and her husband, Michael, thought 6-year-old Mikey's wish would never come true.
"We had a heart to heart one night and decided we would be perfectly content as a family of three," Marotta told TODAY Parents. "When Mikey would ask when he was going to get his baby brother, we would explain he may not get one — that some families only have one child and that is OK. He would tear up and say, 'That's OK if I don't get a human brother; I have Dillinger,' our dog."
But in the summer of 2017, Marotta received what she calls a "happy shock" when she found out she was pregnant again. The Massachusetts couple couldn't wait to tell their son the good news.
"When Mikey found out, he teared up and was very happy," Marotta recalled. "But he was never surprised or overly excited when we found out it was a boy. He just said, 'Obviously I am having a baby brother; it's what I asked for.'"
Marotta says her son worked hard to prepare for his new role as big brother.
"On Halloween, while trick-or-treating, he talked about all the costumes he could wear with his baby brother next year," said Marotta. "He had so many plans. Whenever he had two of something or got something newer, he would say, 'I'm going to save this other one for my baby brother.'"
Mikey had every detail of life with his sibling planned out.
"He is under the impression he has red hair because I ate buffalo chicken when I was pregnant with him, so he kept telling me to make sure I ate buffalo chicken so the baby would have red hair, too," joked Marotta.
But Mikey didn't account for his brother arriving nine weeks early and spending more than 60 days in the NICU.
After being diagnosed with intrauterine growth restriction — a pregnancy complication that affects the baby's growth — Marotta was monitored in the hospital for several weeks before delivering her newest son, Jake, via emergency C-section in March.
"The first time Mikey saw Jake in the NICU, he just stared at him and looked so overwhelmed and nervous," said Marotta. "The first thing he said was, 'He has red hair like me!'"
"We told him what every tube and wire meant — every beep and alarm," Marotta continued. "And Mikey understood how important skin to skin contact was for the baby because we talked often about how it was something he and I did when he was a baby."
So when it was Mikey's turn to safely hold his baby brother — who was just 11 days old and weighed two pounds at the time — he was sure to wear a button-down shirt that let him hold Jake close to his chest.
"He was emotional and nervous, too," said Marotta. "He giggled the whole time — it was so sweet."
And the skin-to-skin bonding was a success — Marotta says today Mikey and Jake, now nearly 5 months old, are unbelievably close.
"When Jake hears Mikey's voice, his little head twists and turns to look for him," Marotta explained. "Mikey was also the first one Jake smiled at, and for a while he was the only one who could make Jake smile."
Marotta recently shared her sons' story on Love What Matters, and says the connection the two boys share is heartwarming.
"Mikey tells Jake he loves him all the time and sings him songs he makes up," said Marotta. "He says things like, 'You are a part of me and I will never let anything bad happen to you,' and he keeps a picture of Jake in his little wallet."
"I feel like Mikey knew in his heart all along this little brother of his was coming — it was just a matter of time."