If a fetus is a lump of cells, how do you explain sonogram?

By the time a woman knows she is pregnant (usually about 10 weeks) the fetus has all major organs in place, and his/her heart is already pumping his/her blood through his/her circulatory system, which is completely distinct from the mother's circulatory system. "Personhood" is a philosophical concept that cannot be objectively defined, but the fact that the fetus is a human being cannot be denied scientifically.