"Not sure if that was to make me feel less huge or just a fun cool theme but I loved it & had the best time ever, feeling so very comfortable eating churros with a bunch of pregnant women!" she added. "Just what I needed, the perfect chill yummy food birthday!!!!"
Kardashian also posted a picture of the all-white cake decorated with white roses and a sign that read, "Happy Pregnant Birthday Kim."
The rapper, 38, brought together Kardashian's loved ones for the special movie viewing at the Westlake Cinepolis theater in L.A.'s Thousand Oaks neighborhood, according to Us Weekly.
But Khloe did break her silence on Instagram to wish her older sister a happy birthday.
Happy birthday Keeks!!" she wrote alongside a photo of the two of them. "I pray you know how loved and dope you are!!! You continuously amaze me by how you do it all. Thank you for always being there for everyone! Today is about you!!! Enjoy it Mrs West!!! I love you, My bestie, my sister, my rider!"
Kardashian, dressed in a beige dress and coat, arrived hand-in-hand with West, who wore cream sweats and white jeans. As they entered the theater, the guests cheered and began singing, "Happy Birthday."
The theater's lobby was decked out in "K" shaped balloons and featured a flower-covered wall. Guests brought gifts wrapped in black and white paper.
Earlier in the day, Kardashian took to Twitter to thank her family, friends and fans for all the birthday wishes.
"I love you guys so much!!!" the pregnant star tweeted. "Thank you for all of the birthday love! It means so much to me!!!!"
I love you guys so much!!! Thank you for all of the birthday love! It means so much to me!!!! ?????? — Kim Kardashian West (@KimKardashian) October 21, 2015