Is there a more divisive issue in American politics than abortion? Here is a timeline of the early stages of pregnancy that helps me think about the issue.
Pregnancy timeline 
Day 0: beginning of the month’s menstrual cycle; this is the beginning of the last period before becoming pregnant; it is where healthcare professionals consider pregnancy to begin (gestational age).
Day 15: sexual intercourse results in fertilization of an egg (ovum); the fertilized egg is called a blastocyst; governments typically begin numbing the weeks of pregnancy from this point (conceptional age).
Day 22: the blastocyst is implanted in the wall of the uterus; home pregnancy tests can detect the presence of the hCG hormone in urine.
Day 25: the blastocyst is called an embryo.
Day 28: first day of missed period.
Day 30: primitive streak is formed; many bioethicists consider the creation of the primitive streak to signify the creation of a unique human being.
Day 35 (week 5): a heart beat can be detected.
Day 42 (week 6): the spinal cord and brain begin to develop.
Day 49 (week 7): organs such as the lungs, liver, and kidneys begin to develop.
Day 56 (week 8): the embryo begins to move.
Day 70 (week 10): the embryo is called a fetus.
Day 84 (week 12): end of first trimester.
Given that the taking of innocent life is morally wrong; the discussion about abortion revolves around the question of when an unfertilized egg cell becomes an innocent human life. I would place this transformation somewhere between day 30 and 49.