Embryo implants but stops developing

Embryo implantation occurs when the embryo nests in the endometrium. This marks the beginning of the pregnancy and leads to the release and, therefore, increase of the hormone beta-hCG. When a chemical pregnancy occurs, implantation and increase of beta-hCG occurs, but after a few hours or days, the embryo's development stops and the release of beta-hCG ceases, which leads to a decrease in its level in the mother's blood.

Pregnancy tests detect the hormone hCG. Therefore, in the case of a chemical pregnancy, the pregnancy test is first positive (due to the increase in beta-hCG) and, after pregnancy loss, the test is negative due to the decline in beta-hCG.