Jessica Simpson 'Had Not Shut the Door' on Having a Third Child Despite Using an IUD: Source

Jessica Simpson told Ellen DeGeneres in May 2017 that she and husband Eric Johnson were “not having a third” child because the two they have are “too cute” and “you can’t top that.”

The musician, actress and fashion designer even went as far as stating, “We got an IUD. Nothing’s gonna get in that uterus.”

So Simpson, 38, shocked fans when she announced on Instagram last week that she and former NFL player Johnson are set to welcome a baby girl and become a family of five.

“They were content with the two and this is a very happy surprise!” a friend of the third-time mom-to-be tells PEOPLE in this week’s issue.

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With son Ace Knute, 5, and Maxwell “Maxi” Drew, 6, both in school now, the pal reveals that Simpson and Johnson, 39, chose to leave the decision to have another baby up to fate.

“They had two kids close in age and they were getting in a phase where both are in school,” says the source. “They would see what God’s plan was — and that’s what happened! They definitely had not shut the door.”

Maxi and Ace can’t wait to have a baby sister. “The kids are super excited,” the insider continues. “[Overall] Jessica is a pretty relaxed person regarding her pregnancies. She wants to enjoy it.”

RELATED VIDEO: Baby Girl on the Way for Jessica Simpson and Eric Johnson

Since revealing her baby news last week, Simpson visited New York City to prepare for the upcoming launch of her makeup line and fragrance.

“She’s feeling really great and they are very excited,” the Simpson friend tells PEOPLE. “With her second pregnancy, she had some nausea, but this time she feels really good.”

For more on Jessica Simpson, pick up the latest issue of PEOPLE, on newsstands Friday.