Weird question...just curious!

There's this girl I work with who is 31 weeks pregnant. She's barely showing at all and can suck her belly in almost completely flat! She is a thin girl and I've never seen or heard such a thing and was wondering if anyone else can suck their stomach in at over 30+ weeks pregnant. Like it looked like her baby disappeared lol. I've never been pregnant before so I wouldn't know, but am actually quite curious!! Lol