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More to the point, life cannot begin at conception, nor can a conceptus qualify as a “person,” as many assert.

The problem with making policy on abortion is this: A conceptus does not appear equal to a person.

If a conceptus has only a single chromosome—a Y—it probably fails to implant in the uterus and never develops.

Normally, when a sperm fertilizes an egg, the resulting human conceptus has 23 pairs of chromosomes.

An important function of progesterone, Fessler said, is that it dials down an early-warning part of the immune system, inflammation, which might prevent the embryo, or conceptus, from implanting itself in the placenta.