I object to the treatment of "abortion" as if it could be discussed and evaluated in the abstract, apart from the circumstances of any particular pregnancy. I therefore object to any law setting the conditions under which an abortion can or can't take place (apart from medical regulation equally applicable to tonsillectomy and abortion) because such legislation can't possibly take account of singular circumstances. I also object to laws restricting abortion (or requiring it) because such laws assume that the individuals involved (particularly but not exclusively women) are so lacking in moral judgment that a legislature wholly uninformed of their circumstances is better equipped to guide their conduct.
Focusing on "abortion" as a topic of debate also obscures what the discussion is--or should be--about. It's a little like calling all discussions of cigarette smoking the pneumonectomy debate. Abortion is a procedure that terminates a pregnancy, but the alternative to weigh against abortion is not childbirth but motherhood. Denying a woman an abortion forces her to become a mother. The discussion should be about coerced motherhood. There are many, many aspects of our culture, our economy and our physiology operating to make motherhood inevitable while representing it as the result of choice. Abortion rescues from motherhood unwilling women who have been swept into it by these inexorable pressures. Denial of abortion is the iron fist of coercion inside the velvet glove of choice.
Motherhood is a life-shaping joy and a wonder, but the magnitude of the blessing is also the measure of the burden imposed on the woman who is not willing to bear it. Under no other circumstances do we impose a comparable burden on unwilling men or women (at least, not without a trial by jury and proof beyond a reasonable doubt). We do not require any man admitted to medical school to spend his life as a doctor, no matter how rewarding a medical career may be or how desperately we need doctors.
As to when "life" begins, approached as a scientific question the determination is irrelevant to the question of when, if ever, motherhood should be required, or to any other moral question. (And "viability," the legal dividing line between impermissible and permissible coercion, is a moral abomination.) Human life begins when a being becomes a member of human society. That may happen before conception when a woman accepts that she will have a baby, or in tragic cases fail to happen even after birth.