Poor women do not want children who will inevitably suffer. If poor children are not born, they will not die. If
fewer children are born, there will be more resources for the people who are already alive.
Unplanned children are also more likely to be the victims of abuse, neglect, and sub-standard parenting. If people don't become parents until they want to become parents, they will have time to become the parents they want to be. Furthermore, as the population is replaced with more and more individuals who were raised by dedicated, competent parents society will become more and more functional, less and less miserable.
Especially in developing countries, educating girls decreases poverty in the short term and reduces family size in the long term. In those same developing countries, educating women--or even just giving them a little bit of money-- helps them dig their way out of poverty. (Give it to the men and more than likely they'll spend it on alcohol and/or prostitutes, sadly.)
That's why I give a lot to Planned Parenthood International and Planned Parenthood Federation of America.
Women for Women International helps women and mothers and their daughters in developing countries.
See Amazon.com: Half the Sky: Turning Oppression into Opportunity for Women Worldwide (9780307387097): Nicholas D. Kristof, Sheryl WuDunn: Books