Before Michelle Monaghan went public with news that she’s pregnant with her first child, her Made of Honor costar, Patrick Dempsey was already aware of her condition.

“It was a big secret, and I wasn’t allowed to tell anyone, but I knew Michelle was pregnant, and I am thrilled for her,” Dempsey, 42, told PEOPLE on Friday – as he prepared to race this weekend in the Six Hours of the Glen Grand-Am Rolex Sports Car Series at Watkins Glen International in upstate New York.

“She will be a great mom because she has a good head on her shoulders, a fine husband, and overall, she is a lovely woman,” he said.

Dempsey, who with wife Jillian has 18-month-old twin sons (Darby and Sullivan), and a 6-year-old daughter (Talula), gave Monaghan, 32, some advice on being a first- time parent.

“I told Michelle she may not see it now, but her life will change profoundly after she delivers the baby,” he recounts. “I told her being a parent is a feeling that she will not understand until she actually has the child.”

Dempsey, who races as often as his acting schedule allows – he’s due to report back to the set of Grey’s Anatomy June 25 – is looking forward to one day bringing his entire family to the racetrack with him.

“Jillian always tells me to be careful when I leave the house for a race,” says Dempsey, who is competing for the first time at Watkins Glen International. “It is hard to leave the family on weekends because I love spending time with the kids, but I also love to race. I count the days until they can come with me, and we can all be here together.”