ABANDONMENT VERSUS INFANTICIDE

I've noted before that John Boswell's Kindness of Strangers provides a great deal of evidence that the early Church approved of the abandonment of unwanted newborns and according to Emily Coleman¹, this was because it wanted to reduce the frequency of infanticide:

Infanticide had been a practice of "civilized" societies long before the medieval era; Greece (probably Sparta rather than Athens) had institutionalized it, and Rome continued it. Roman imperial laws had attempted to outlaw this practice, and as early as 315 Constantine, recognizing the importance of the economic factor, tried to organize a charity for poor parents so that they would not have to expose or sell their children. In 318 he set the penalty at death for infanticide.  Infanticide was not unknown among the Germanic tribes either although apparently it did not reach the proportions among them that it did among their more sophisticated neighbors.  Naturally, the Church eventually became involved. From the fourth century on, many children were left abandoned at a church's door; and they were accepted in order to prevent their death at the hands of their parents.  However, the Church admitted that there was often an economic necessity behind it. Among the lists of punishments in the penitentials for the killing of infants is an illuminating clause indicating that the penance would be reduced by more than half (from 15 to 7 years) if the mother was a poor woman. 

¹"Infanticide in the early middle ages" in Women in medieval society, Brenda Bolton; Susan Mosher Stuard; et al [Philadelphia] : University of Pennsylvania Press, 1976.