The model and actress, who announced her pregnancy in January, explained that her due date next month is starting to preoccupy her.
"You get big and you get uncomfortable," Sims told Us Weekly.
"You start thinking that at any moment you might go into labour even though you know it's not true."
Sims also joked that her husband Scott Stuber, whom she wed in September, may struggle to cope with the birth.
"We did our first birthing class the other day," she said. "I swear to God I thought he was going to faint!"
Sims, who has confirmed that she is expecting a boy, celebrated her pregnancy with a baby shower earlier this month.