In the human female, at least 17% of the body has to be fat before puberty will occur. The onset of menstruation will also be delayed unless the body contains at least this amount of fat.
It's also important to realise that if a woman suffers from anorexia or one of the other eating disorders, and allows her body fat to fall below this figure, menstruation will stop.
So why does this happen? Well fat is responsible for the production of some vital reproductive hormones. An area of the brain inside the hindbrain constantly monitors the amount of these hormones in the blood. If it detects deficiencies of these hormones, as can happen in anorexic or starving third-world women, it switches off ovulation and thus menstruation.
So why do these women still manage to conceive? The answer is that the biochemical system has a degree of tolerance built in. Every woman is different and one may conceive whereas her neighbour of the same weight might find it impossible. Also, you need to take into account that although you see these women with babies, you don't know how long they've been trying to conceive - it could be many years because these hormones can decrease fertility gradually and one day these women might just might be lucky and "catch", as my wife puts it..
Remember also that its commonplace for women in these countries to start reproducing from a relatively early age. A twenty year old may have been able to conceive for some nine years and even emaciated, could have at least one child if she was lucky. The mother would still look like a teenager in build and stature.