During conception, how do sperm, egg, soul, karma and Krishna’s will inter-relate?

When couple try to mate to have children they may not have children always but only by Krishna’s blessings they do. How does that work? meaning semen may have soul but cannot turn into a baby in the womb of mother. Also some times prematurely, soul leaves meaning miss carriage? what’s happening? Of course there is a karma factor but its not very clear in this case. please explain