"My husband and I are so excited to begin the wonderful adventures of parenthood. We couldn't be happier!" Miller, 32, who's married to prominent Florida Republican and businessman John Falconetti, tells PEOPLE.
The Olympic champ, who won medals in the 1992 and 1996 Games, has focused in recent years on raising awareness to such health issues as cervical cancer – her mother was diagnosed with the disease last year – and says, "That passion has become even more critical and more personal now that I am considering the health of my child."
"I already feel that incredible 'mama bear' protectiveness toward our baby," she says. "Each day I wake up wondering what our baby looks like today, how much he or she has grown, and anticipating the fun and surprises that lie ahead."