used in medical schools acknowledges that a fetus is a human being, quite unlike a gamete, such as a sperm.
Sperm are gametes, specialized cells specifically designed for reproduction. They contain half of the chromosomes (23) of the father. When they unite with oocytes, gametes from the mother which also contain half (23) of her chromosomes they combine into a new unicellular human being with 46 chromosomes. That is an ontological change, that brings a new human being into existence.
This is not a religious belief, it is an observable scientific fact.
If interested, you can read a more detailed explanation at:
http://lifeissues.net/writers/irv/irv_01lifebegin1.html
Scroll down a little bit to the section on
1) Gametogenesis