How many days are syrian hamsters pregnat??

i have a syrian hamster and i bred her and she did get a little bigger but i could never tell if she was actually pregnant but she was due 2 days ago so today would be the 18th day of her pregnancy is it still possible for her to give birth or is she not pregnant?? and do they always have to give birth at exactly 16 days after they mated??? thank you sooooooo much!! , jess