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!"
Sims, who at 43 is considered "high risk" for giving birth, said that it doesn't worry her that much, as she's been "high risk for the last ten years" during both previous pregnancies.
Sims and Stuber welcomed their first child, son Brooks, in June 2012 and daughter Scarlett in March 2015. As her third is on the way, Sims said she finds herself asking, "How did this happen?"
"[We're] like, 'Why are we doing this!?'" Sims joked. "We were man-on-man, now we are going to be outnumbered!"
In her pregnancy announcement video, Sims admitted that the baby news came as a shock to her and her husband, who "never expected it would happen."
"This was such a true surprise," Sims shared at the time. "I think that's why I'm so emotional."
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, the one name Sims really wanted to use was already claimed.
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.