The immediate explanation that comes to mind is that taking care of a baby aroused their maternal instincts and changed their hormonal status and made them more pliable to conceive.
I have often wondered whether it is not such obsessive self doubt about one's ability to control hostile impulses towards one's children that lies behind so many women's inability to conceive. This factor also may be present in those men who never venture into parenthood or at least postpone it till late in life.
I wondered too if such obsessive doubts about being a good parent does not lie behind Post Partum Depression. Here of course the baby is one's own and already born, nevertheless, the fear that one could harm it prevents one from taking up the role of motherhood. The psychodynamics of post partum depression is, of course, more complex and the factor of sacrificing one's child to one's own mother - here one is reminded of the Hindu practice of sacrificing one's child to Kali, the mother goddess, for propriation and prosperity- for getting something even more precious in return, comes in as well. Nevertheless the fear that one may harm one's child lies behind post-partum depression too. The depression is triggered by unconscious defense mechanism to put a lid upon the impulse to harm.