Don't ever use the term "baby" to describe a foetus. You can semantically use the term "unborn child", but don't be scared to use the term foetus.
Them, you, me, all once a foetus. Its a stage of human development. An embryo becomes a foetus becomes a baby becomes a toddler et cetera, but always a human being. It can be nothing else. To deny its humanity is to deny common sense and medical science.
Depending on these people, you have to select your argument.
Ask them when they think the law should offer protection to the foetus. At some abritrary point in pregnancy? Viability? If they say something during pregnancy, say 25 weeks, ask them why they choose that point? Isn't it still the woman's body at 40 weeks? What about 24 weeks and 7 days?
IF they're one of the "abortion on demand, all throughout pregnancy and without apology" crazies, you gotta tread carefully. However, just point out there's no moral/developmental difference between 40 week old foetus and newborn baby, then point out that sentience doesn't really develop fully until about 2 years of life.
They might pull the biological dependance card, ask them why you can't walk into a GP's office and get your leg amputated. After all, its just "Human tissue", its non-sentient and is dependant on your body for resources. The whole "keep your laws offa my body" is a good one to tinker with. Smoking, drinking, voting, joining the army, all are laws that govern what you can do with yoru body, preventing you from using it in a lot of cases, why should the law allow you to kill another person's body?
Fifty five million children, that's 14 times the population of my country. Abortion is a tragety. [like my spelling]. Try pointing out the logistics of such numbers, that's a 1/3rd of our generation, ask them how 2/3rds of a generation is going to support 3/3rds of the older generation when they hit retirement age.
Sometimes you just have to pick your battles though. I've been doing the online abortion debate thing for 15 years now. I can safely say I've seen every pro-abortion argument there is, and when I FB stuff, people don't engage me for very long. They're too scared of me. -_-' I personally find that irritating because I like a good challenge and debate.
The best thing you can do, and I've seen it suggested in Catholic apologetics, is turn the argument back on them, don't always be on the defensive. The Pro-abortion argument is illogical and quite frankly, chilling. Once they're on the spot, they either show their true blood covered colours or their ignorance of the issue, and usually, somewhere amongst all that confusion, they start to realise something about themselves that's not so pleasant.