Yes, pregnancy test becomes negative after 16-20 weeks of pregnancy. This due to the fact that all pregnancy kits detect presence of an hormone in urine or blood called hCG that is secreted by embryo. This hormone is used to develop placenta and also maintains corpus luteum that secrets progesterone. This progesterone maintains pregnancy. When placental development is completed then hCG is no longer needed because placenta itself starts to secrete progesterone in huge quantities. So, hCG is no longer produced and pregnancy test becomes negative after 4-5 months of pregnancy