Well this is what I have learnt. In Buddhism those who die young or those who die even before they are born have done past karma related to taking the life of others/intentional killing. That is why Buddhists have "abstain from harming or taking the life of others (humans and non humans)" as the first percept in their 5 percepts. Those who intentionally harm the lives of others or intentionally encourage another person to harm the lives of others will be born with a shorter life span. Hence ,whether they have completed the purpose here or not their life will be taken same as they did in the past lives for others. Those who suffer due to diseases have caused physical harm to living beings. Those who suffer from mental agonies have given psychological/emotional pain to others. It is only our own actions in the past which bring challenges to us. You reap what yo sow. In the case of miscarriages it is the karma of the child and that of the parents. It sometimes be because the child simply doesn't belong to that family but somewhere else.