@asdfdsa and bob:
And where is the woman in your logic? She is entitled to her life and her autonomy of person, and should not be coerced to be an incubator. We don't force people to donate blood or kidneys or lungs. The personhood or not of the fetus is irrelevant.
Also:
If abortions are illegal, they still happen at the same rate - the difference is that they are simply unregulated and unsafe. And then women die. How many women should bleed out from uterine hemorrhage or die from infection or because their kidneys shut down just to make you feel better? Or do you think that women should die because they cannot be or do not want to be pregnant?
Finally:
Do not mistake pro-choice for "pro-abortion". Abortion is necessary, but it should be necessary as infrequently as possible. This is an economic argument as well as a public health one. It is far cheaper (by maybe a factor of six) to prevent unplanned / unwanted pregnancies than it is to terminate them. The abortion rate in the US can be dropped by up to a factor of twenty by improved contraceptive use - most of the remainder are medically-necessary abortions where the fetus would not be carried to term anyway (that factor of 20 is an upper limit if everybody is on strong contraceptives; real-life contraceptive uptake rates in Western Europe are high enough to bring the abortion rate down by a factor of 2 as compared to the United States).
So I assume you both will approve universal and affordable access to effective contraceptives and comprehensive sex ed so that everybody knows to use them and how to use them? That's the only guaranteed way to reduce the rate of unplanned pregnancies and so reduce the rate of abortion. If you don't, then you aren't being honest when you say you want to prevent abortion.
Post-script at asdfsa individually:
The world is overpopulated.
Right now, 35% of the potentially cultivated land in the world is being used to grow crops, and as much or more fish are being fished out of the oceans than is sustainable. This, the Green Revolution, and lots of artificial fertilizers let us feed 7 billion.
Could we feed more people? Yes. But to do so with current food consumption would mean taking over even more of the ecosystem, with unknown and likely very unpleasant results. We could all become near-vegetarians, which would raise the number of people we could feed somewhat.
But access to clean water, good education, good medical care, and all of the conveniences and diversions of much of current developed-nation global society would be impossible in a world with a population of 18 or 20 billion. There simply isn't enough space or energy.
The question becomes what individual standard of living we want. That determines the sustainable world population - the carrying capacity. Estimates with the current distribution of standard-of-living are perhaps 4-5 billion. If you want everyone to live like people do now in Western Europe, the number is more like 3 billion; like the US it's a bit lower still. Improved technology could raise those numbers, but not to much above the current population.
Unless you want everyone to be struggling to survive in our collective waste, the population needs to at least level out or, preferably, come down gradually over the next hundred or so years.
Post-script at bob individually:
You Fail Biology Forever with that argument.
A tumor meets the definition of life you are using, and has different DNA than the patient it is growing in. But we still treat cancer by killing the tumor cells (see also Felix's comment below on the definition of 'parasite').
Identical twins have almost the same DNA, but are two different people.
You can make an argument based around the presence of a functioning and well-developed central nervous system, but almost all abortions occur before the fetus develops one. Those that don't are either because of artificial delays in the procedure being performed or, more commonly, medically necessary where the fetus is going to die anyway.