From a biological standpoint the purpose of offspring is to carry on the species... not the other way around. If having offspring in a particular situation actually hinders the long term viability of that family (as a subset of the species) to carry on then it is counterproductive. The appearance of a new baby in some situations can destroy chances of the mother moving up economically and educationally and socially/emotionally - and thus hurt her and her offspring and potential offspring in serious and long-lasting ways. As a mother would you choose having a kid when you are 18, drop out of school, struggle with low-paying jobs, warehouse the kid in cheap childcare, not spend much time with the child, etc. and never be able to provide anything more than mere subsistence for the child or would you choose aborting that pregnancy, seeking counseling, go to college, get a good career, marrying, have two or three kids at about 30, share child-rearing, provide quality pre-school, live in a better neighborhood and therefore probably a better elementary school for your two kids, etc... Which is better? Which is better for the family and society? Which builds towards a better future?
My wife got an abortion when she was young and still living with her mother... she still reflects on it and it saddens her - but she also never has regretted it. Her life would be nothing like it is if she went through with bearing that child. And the life that she has been able to provide - the loving, happy, attentive household for our two children would never have been possible... and her successful career as a teacher (an excellent teacher) would not have been possible. She touches the lives of many people in ways that wouldn't have happened if she was stuck at some dead-end job alone and "raising" a lonely, poorly educated child.