They are fertilized ova, the beginning of human life.
Prenatal diagnosis, which presents no moral objections if carried out in order to identify the medical treatment which may be needed by the child in the womb, all too often becomes an opportunity for proposing and procuring an abortion. This is eugenic abortion, justified in public opinion on the basis of a mentality--mistakenly held to be consistent with the demands of "therapeutic interventions"--which accepts life only under certain conditions and rejects it when it is affected by any limitation, handicap or illness.
Following this same logic, the point has been reached where the most basic care, even nourishment, is denied to babies born with serious handicaps or illnesses. The contemporary scene, moreover, is becoming even more alarming by reason of the proposals, advanced here and there, to justify even infanticide, following the same arguments used to justify the right to abortion. In this way, we revert to a state of barbarism which one hoped had been left behind forever.
Evangelium Vitae
An adult is just a bunch of cells on a couch. A newborn baby is just a bunch of cells in a cradle.
How big a bunch of cells does an individual have to have in order to be worthy of life?