At 13 weeks the fetus is around 6.7cm (2.6 inches) long and weighs as much as a pea pod! Granted there is placenta and fluid and other things (bloating!) happening at this point, but at 13 weeks you should maybe be able to tell she's pregnant.  If it's her first kid she will also show later. Here are some ladies at 13 weeks: One and two Edit:  beaten by 3 minutes.