(Cover) - EN Showbiz - Hilary Duff is exercising with a celebrity fitness trainer to stay in shape while pregnant.

The 23-year-old actress announced that she and professional hockey player husband Mike Comrie are expecting their first child together last month. Hilary has been working with world class fitness expert Harley Pasternak three to five times a week to keep fit in her first trimester.

Harley has developed a personalised exercise programme for the star.

"[We do] no impact, no hyper bouncing and plyometrics," Harley told People magazine. "You also want to back off stretching quite a bit. Women become hyper flexible when they're pregnant."

Harley and Hilary practice in a cool room and her heart rate is always monitored. Her cardio is low impact, she does elliptical, walking and stairs occasionally. The pair avoid all routines focusing on Hilary's abdominal region.

"In the second trimester, [we're staying] away from any exercises where she's lying on her back and that could put pressure on her midsection," Harley said.

"We just go lighter and higher reps on certain exercises. So there's nothing pressing against the stomach and breasts."

Harley believes that Hilary's baby will be just as fit as she is. He is also convinced that she will be an excellent mother.

"Hilary's fit, young, [and] she's healthy. So I think it's going to be a great, beautiful, fit baby." Harley said. "[She'll be] an amazing mom!" (C) Cover Media