"Showing" occurs at different intervals for people, and while it's hard not to compare pregnant bellies, it really is a good policy to avoid comparison. A mother who has had previous children will likely show more quickly in her second pregnancy (or more) than a mother pregnant for the first time. Also weight gain, previous weight status (slim, normal, overweight), whether or not you've had previous abdominal surgery, the tone of your muscles etc. will all play into account. That said, many women start to feel the need to be out of their regular clothes by the 3-4 month range. This doesn't necessarily mean maternity clothes for everyone at this point.