We put ds1 onto a matress on the floor at 18months 1week lol and when his bed was deliverd 1week later he went into the bed with the rail, he has never fallen out and he never got out either, well until the last few months, he gets out now occasionally but he is a cheeky 2.5yr old!

Also it was because I was due with ds2 mid april so we gave it extra time so the baby wasn't stealing his bed iykwim? I would look for things like climbing, if you think she could climb out I would put her in a bed or if she physically was too big etc