How common is decidual bleeding (percentage wise)? how common is a negative hpt test 3 weeks after period?

High ; common: Decidual bleeding can be lumped together with implantation bleeding and we can call them both "first trimester bleeding". I'd estimate 20-30% of women have some type of first trimester bleeding, and many go on to have normal pregnancies. We usually discourage pregnancy tests until the period is due/late because they are often negative until 2 weeks after conception (or 4 wks after the last period).