A chemical pregnancy is a very early pregnancy loss which takes place soon after implantation- usually before the fifth week of pregnancy. In the majority of cases, it occurs prior to the appearance of a gestational sac on ultrasound and the only indication of pregnancy is a transient elevation of the hormone human chorionic gonadotropin. https://www.fertilitywhisperer.com/blogs/what-is-a-chemical-pregnancy-causes-symptoms-early-signs-explained/