It varies with every woman. There's no exact day or week.

As justcurious said, it will vary, and it can vary greatly.

Generally most women will start to show somewhere in the second trimester. You may notice a bit of a bump earlier on, but others may not notice for awhile. It can also depend on your build and how many pregnancies you have had.

To add to what justcurious and Doula said... it also depends on the position/tilt of your uterus.

I have known a few women of slim stature to never show at all due to the posterior tilt of their uterus.