I am personally against abortion, but I also acknowledge the need of it given we have no other forms of population control. We are struggling with the population we have now, it would be far worse with several billions more people on top of it.
As for the "when life begins" debate, really the only way I can think to see it is that at the moment of conception what happens is a series of developments that do not stop until the organism ceases to function entirely. Be it due to a natural occurring miscarriage, death in old age, or any other reason, all of life is only one development after another from the time of conception to the time of death.