Katherine Heigl "can't stop thinking" about doughnuts when she's on the set of 'Doubt'.

Katherine Heigl

Katherine Heigl

The 37-year-old actress is pregnant with her and her husband Josh Kelley's first biological child and has revealed that her cravings have been getting the better of her when she's filming the upcoming legal TV series.

Speaking to 'Entertainment Tonight', the star said: "I feel like what I need to do is just have one doughnut a week. Just have one, and then stop thinking about it. Here's the problem, if I'm not on set I don't think about them that much and I don't eat them because they're not around. But on set [there's] always a plethora of doughnuts and I can't stop thinking about them."

But the 'Knocked Up' star - who has two adopted daughters Naleigh, seven, and Adelaide, four - "loves" being pregnant as she can use it as an excuse to eat junk food.

She said: "The sweet cravings [are] all the baby's fault! It's lovely actually because I feel absolutely no responsibility for my behaviour at all. It's great."

However, being pregnant on a film set comes with more issues than just doughnut cravings, as the crew have had to come up with ingenious ways of hiding the blonde beauty's baby bump.

The 'State of Affairs' actress said: "Today was really impressive. I had to come up to my desk and I didn't know that they had strategically placed [a desk lamp]. They were like, 'Katie, would you mind taking a small step to your right?' And I looked down and this lamp was right there. That's clever."

Katherine also admits that whilst she prides herself on being able to memorise her lines, she's found that her "pregnancy brain" has caused her to struggle getting her head around some of the legal jargon in the script.

She said: "Usually I'm pretty good at memorising. I can do that pretty quickly. It's like the only thing I've got. So it's really been eye-opening to me to struggle with that."