At the end of season seven, we learn Cersei is pregnant, which would seem to discredit Maggy's fortune, but a fourth child would be the littlest brother of all her children, leaving Cersei open to danger. Reddit user Bioreactivist believes death by childbirth is the key to Cersei's story arc: "It affirms that her hatred and fear of Tyrion has been entirely unfounded, and would be a poetic end for her character — to die in the same manner as her mother, for which she has always blamed her little brother. Lastly, it would fit in well with GoT's ever-present theme of vanity and futility, as all of the death, destruction, and hatred that she has committed out of fear of this prophecy will be proven to have been ultimately pointless."
The only thing that could make this theory more ominous is if the child turned out to be a dwarf like Tyrion Lannister.