Sims announced in August that she and her husband, movie producer Scott Stuber, were expecting their third child, sharing the happy news in a video posted to her YouTube channel.
Since then, however, the Las Vegas star says she's been struggling with "nausea, motion sickness, heartburn" and general lethargy, adding, "It's been tough."
"There have been a couple of days [when] I was like, 'I am not going to make it,'" Sims told ET at the America’s Messiest Baby Contest, presented by Dreft Baby Laundry Detergent at Maman Tribeca in New York City on Tuesday. "But we are so happy. I just want everything to be good!"
As it turns out, the reason it was such a bombshell is that the couple weren't actually planning for another kid, but were simply having a bit of fun after being apart.
"I went to visit my husband on [the set of his movie] Office Christmas Party, with Jennifer Aniston and Jason Bateman," Sims admitted with a laugh. "I went to Atlanta to visit him [because] I felt guilty for not going, and there you go!"
"It's our Office Christmas Party surprise," she continued, adding that the movie, which is due out Dec. 9, will be hitting theaters "around the time I'm due."
The couple have yet to find out the sex of their forthcoming bundle of joy, but Sims says they're already brainstorming names.
"I don't want something just to be different. I want something that is meaningful and that I am going to love," she explained. "If it's a girl, I always loved Ruby or Marlo or Stella. And for a boy, I love Westley and Finley and Wilder."
However, whether it turns out to be a boy or a girl, Sims is adamant that this will be the couple's last baby, and she's going to demand her husband take necessary precautions.
"If he doesn't get a vasectomy, I am going to take him. He is getting one," she insisted. "He doesn’t want to, and his friend had a bad experience, but I don't care. He's getting it. This is it."
ET caught up with Sims at the Autism Speaks "La Vie En Blue" Fashion Gala in Los Angeles late last month, where the actress opened up about expanding her "tribe." Check out the video below to hear more.