The actress welcomed her first child - a daughter named Harper Renn - with husband Brady Smith in June.
"I'd love to be my size again but my body is just not that way," she told People.
"It doesn't work that way for me and I'm okay with that."
Thiessen revealed that she gained 50lbs during her pregnancy and is still working on losing the final 20lbs.
"When you're running on four hours of sleep it's a little hard to get motivated but I am definitely trying," she explained.
"I'm going to the gym three to four times a week and I started to run again."