If you haven't been pregnant, you can't really know what those nine months are like. Your body is simultaneously your body and not your body. Your mind is simultaneously your old mind, and some strange, bizarre thing that is transformed by hormones: emotional, irrational, capable of new things and incapable of old ones. The fetus is simultaneously part of you and something of its own, and changes throughout those nine months, from something indistinguishable from the experience of bad PMS during the first month (and how many menstrual periods may involve the natural self-aborting of a fertilized egg, or the failed implantation of an egg?), to something that wriggles and kicks and responds to music and movement. It is a terrible and wonderful experience. Nobody should be forced to experience it. Nobody should be forced not to experience it. And the physical and mental experience does not end with the separation of the fetus/child from the woman, whether the pregnancy ends with birth or with abortion or with miscarriage; whether a child is adopted or raised by its birth mother.
There are no easy answers. Life does not have a simple beginning. An egg is a living cell. A sperm is a living cell. A fertilized egg will not live unless it is implanted. Without intervention, some fertilized eggs will go on to become fetuses and then babies; many will never be successfully implanted, or will be miscarried, or will be stillborn. With medical intervention, some fertilized eggs that would otherwise not have survived will go on to become adult human beings. Intervention may prevent some eggs from being fertilized, or it may prevent some fertilized eggs from being implanted, or may prevent some fetuses from being born. And human intervention may prevent some children from living to adulthood, or may keep some children alive. Humans keep some young men and women alive when they would otherwise have died, and kill others who would otherwise have lived. Humans can keep a brain-dead woman alive on a machine, and can use machines to kill convicted criminals.
"Thou shalt not kill" is a far more complicated commandment than it may seem at the surface.