The 42-year-old actress - who plays Maddie Buckley on the show - has revealed she pushed herself to her physical limit in order to make the pregnancy scenes look as realistic as possible.
She explained, "With both of my children, I laboured for a long time, but I pushed only twice, so I got very lucky to only push a couple of times, so I never pushed for like nine hours straight, so it was a lot, I had a very bad migraine, I broke a lot of blood vessels in my face."
One of the medical experts on set joked about the realness of Jennifer's performance.
The actress - who has Autumn, seven, and Atticus, five, with Brian Hallisay - told Entertainment Tonight, "One of the medical experts on the show at one point she was like, 'If a baby doesn't actually come out of there right now, I am going to be amazed.' "
"I was like, 'You're telling me sister!' So it was a lot, but the audience is going to love it. It's really exciting and it's a fun episode for Maddie and Chimmy."
Despite this, Jennifer admitted she didn't give a lot of thought to the pregnancy scenes before the cameras started rolling.
She shared, "Maddie does end up in labour in a very inopportune time cause she really wants to push through and work right up to the last minute. She has no idea that last minute is going to be during a 911 call."
"A very serious call by the way and it was really funny in filming because I was like, 'How does one prepare to go to work and do this scene,' you know? Normally you try and think about these things, and I was like, 'I think I just have to not think about it. I think I just have to do it and see what happens.' "