Jill Duggar Dillard and husband Derick, stars of TLC's hit reality show 19 Kids & Counting, are expecting a son in the spring, they tell PEOPLE.
"I was really excited," says Derick, 25. "As a new dad, to have a firstborn son to be our first child and be the leader of his siblings, it is very special."
The couple had a little company at their gender-revealing ultrasound last Saturday – Jill's siblings Joy, 16, James, 13 and Jenny, 7 – who each had his and her own guess about whether a girl or a boy would be joining the family.
"Oh yes, there was some screaming in the room," says, Jill, 23, who is 18 weeks along as of Tuesday. "I thought all along we were having a girl. Everyone made their guesses."
To make sure the baby was really active for his first big close-up, the Dillards prepared for the big moment with a breakfast of waffles with syrup and whipped cream, fruit, chocolate milk and lemonade.
"He stretched out his legs full length," says Jill. "He was trying to be like Dad."
Jill married Derick on June 21, and revealed her pregnancy to PEOPLE weeks afterward. This year's 19 Kids & Counting season features her wedding plans – and a sneak peek of Tuesday's episode (below) shows that Jill, a midwife, is already around many babies at home.
"I was praying for a big, healthy baby," says Derick about the night before their ultrasound appointment. "And Jill added, 'Just healthy, healthy.' "
"We could have a giant," explains Jill. "I called Derick's mom and she was telling me that when Derick was born, he didn't fit into the newborn clothes and his shoes were a size two."
For now, they're shopping for items like overalls, plaid shirts and pants, as well as narrowing down names for consideration.
"We are not going to do a letter theme," says Jill, whose parents, Jim Bob and Michelle, all have children with "J" names. "But we may do some kind of theme, like family names."
And what did Jill's parents think of the news?
"They were just elated," says Jill. "My mom and dad both thought it was going to be a boy and my grandma did, too. They are all really excited."
Tune in to TLC Tuesday for an exclusive look at Jill and Derick's reaction to their recent baby news.
For more on the couple's baby plans, pick up the latest issue of PEOPLE, on newsstands Friday