The model is expecting her third child with husband Scott Stuber. The new addition -- which will arrive at the end of the year -- will join the couple's son Brooks, 4, and daughter, Scarlett, 16 months.
"It's a Hail Mary!" the 43-year-old actress tells PEOPLE of her pregnancy. "I'm so happy — we're so blessed. We were so shocked, but we're thrilled."
Sims tells the mag that the pregnancy was unplanned, joking that it was the result of "what happens when I go visit my husband on the movie set."
"Being 43 is a little worrisome, cause I'm like 'Okay, you are going to be 90!' But then Mick Jagger [is having a baby at 72] so we're like okay, we feel good about ourselves," she adds.
While the mama-to-be is thrilled about her bundle of joy, the pregnancy has had its ups and downs.
"I have never had more morning sickness in my life — I have been sick from the moment I wake up to the moment I go to bed," she says. "I have thrown up in the Bristol Farms parking lot and I have thrown up outside of a restaurant. It's just bad!"
Morning sickness aside, we can bet this baby is going to be adorable!