The 40-year-old singer gave birth to Max and Emme in March 2008. Before they were born Jennifer had no experience with babies, and had to be coached through nappy changes by her husband for the first few months because she was so scared she would hurt the tots.
Marc already had two children with his previous wife Dayanara Torres, and was happy to help Jennifer learn to look after their new babies.
"He had to teach me the basics of diaper changing," Jennifer said. "I was almost afraid to touch the babies at first because I might hurt them, because they were only 5lbs each."
Jennifer is now a "completely different" person. She adores her children, and finally feels like a proper adult with her own family.
"I can`t help but be a different person now that I`ve had kids," she told the May issue of Redbook magazine. "That really does change your whole perspective on life for the better. I definitely feel like I`ve grown up.
"My main priority now is that my kids are happy - that`s my number one focus in life. Are these little people happy, content, and getting everything they need? Everything after that is secondary. Before, my work was my main priority - even above myself."