Can you be three weeks' pregnant and have a miscarriage?

Definitely; the majority of miscarriages occur within the first 12 weeks of pregnancy. Fifty percent of women who miscarry don't notice and continue without ever noticing. A miscarriage may occur before the end of the first trimester if the fetus is not developing normally. Although a miscarriage can be frightening and upsetting, an early miscarriage is most likely nothing to worry about.