We. We date pregnancies from the last menstrual period lmp. At 2 weeks from the LMP you would be ovulating. At 3 weeks you are unlikely to have symptoms and only sophisticated blood testing is reliable for diagnosis. By 4 weeks most home pregnancy tests will be positive. Some early signs of pregnancy 2-3 weeks post conception (4-5 weeks gestation from lmp) are fatigue, breast tenderness and nausea.