'I gained 40 pounds': Katherine Heigl reveals her 'terror' as she packed on the pounds during her pregnancy with newborn son Joshua

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Katherine Heigl and husband Josh Kelley welcomed son Joshua Bishop Kelley, Jr on December 20.

And the 38-year-old actress explained the 'heartbreak' she felt when visiting the doctor's office each month, seeing how much weight she gained.

'I gained like 40 lbs,' she told People. 'That’s hard for an actress who’s spent her whole life trying to always lose that last 10.'

The Knocked Up actress, who gave birth to her first child, explained: 'What was rough with the pregnancy was just the terror of the weight gain.'

Precious! Katherine Heigl revealed the first photo of her newborn baby boy in that February issue of People magazine and says she gained 40 pounds during her pregnancy

'I tried to be as healthy as I could about it and not go crazy and go over the top with weight gain, but I still gained a lot for me.'

The blonde beauty described other physical changes from breastfeeding as she continued: 'My boobs just got so ginormous.'

'It was horrifying! I’ve always had fairly large breasts, but [I went from a] DD to an I. Did you know that an I even existed in bra sizes? I didn’t,' she exclaimed.

'I didn’t know that until I started having to buy bra sizes in an I. That’s D, E, F, G, H, I -that’s horrifying!'

'Incredibly grateful': The Knocked Up star took to Twitter on Wednesday and sent a heartfelt thanks to her fans

Proud parents! Katherine Heigl and husband Josh Kelley, 36 - pictured December 2014 -  welcomed son Joshua Bishop Kelley, Jr on December 20

However, the Grey's Anatomy alum admitted, 'But I’m so grateful for my body right now because I feel like it’s doing what it needs to do.'  

And the happy couple finally revealed the first photo of their baby boy in the latest February issue of People magazine. 

'It was a surprise! I knew I wanted to expand our family, I knew I wanted to have more children,' the actress who was busy working on her new series Doubt explained. 'I just didn’t know how we were going to go about that.' 

'I’ll be totally honest, my first reaction was "Oh s***," but now we have a new normal, and I have to give myself a break in order to get used to it,' Heigl added.

'Girls fawning over him': Katherine and Josh adopted their first daughter, Naleigh, 8, from South Korea in September 2009 and another little girl, Adalaide, 4, from Louisiana in April 2012

In June 2006, Heigl became engaged to singer Josh Kelley, 36, whom she had met a year earlier on the set of his music video for Only You and tied the knot on December 23, 2007, in Park City, Utah.

The couple adopted their first daughter, Naleigh, 8, from South Korea in September 2009 and another little girl, Adalaide, 4, from Louisiana in April 2012. 

'This little boy is going to have all these girls fawning over him,' the Knocked Up actress gushed.

Back in June when the blonde beauty announced the exciting news, in a statement, she wrote: 'The Kelley clan is thrilled to announce that we are expecting a third addition to our family.

'Naleigh and Adelaide could not be more excited to welcome their new sibling into the fold and Josh and I are overflowing with joy and gratitude.'

'The Kelley clan is thrilled': Back in June the Knocked Up star announced the exciting news with a statement 

She continued: 'This is an incredibly exciting time for us filled with hope, anticipation, and hormones. Well, only I’m filled with hormones, but everyone in the family gets to enjoy them.

'We couldn’t be more excited to be able to finally share this news with our fans and are looking forward to continuing to share the journey with you all. xoxo, Katie and Josh.'

The two-time Golden Globe nominee didn't always see pregnancy in her cards and said pregnancy 'didn’t ever really speak to me. I got really invested in adoption. I didn’t look back or even think about it.'

'Only when I was about 35 did I start going, "Well, should we consider getting pregnant?" You don’t want the choice to be taken away from you,' she explained.

Legal drama: Meanwhile the Emmy Award-winning starlet is gearing up to promote her upcoming CBS series Doubt next month, co-starring Laverne Cox

Meanwhile the Emmy Award-winning starlet is gearing up to promote her upcoming CBS series Doubt next month, co-starring Laverne Cox. 

In the new legal drama the Orange Is The New Black star plays Cameron Wirth, a transgender Ivy League-educated attorney.

She will star opposite Katherine who will play defense lawyer Sadie Ellis. The TV film is set to premiere on February 15.