Alright, so. You know how Targaryens had a lot of miscarriages, stillborns or children who didnt manage to reach adulthood because of health problems? Yea, i think thats tied back to them abandoning the old valyria traditions and blood magic.
So, we know that valyrians practiced blood magic, right? Now, lemme remind you of the horse eating tradition that the dothraki has. I think that tradition came from the valyrians making their pragnant women eat dragon hearts to determine if the baby will be strong. Thats how i think the tradition came from. Also, i think it was also done to Impregnate a woman with a dragon. Now, i may sound insane for this, but hear me out.
Theres this one Targaryen girl aerea. She was a rider of balerion and when she flew her for the first time, she Dissapeared for a year and returned completely obliterated. Now, when a maester or a septon was treating her, he dumped her body in a cold,icy water to cool her body. But this sent her body into a shock and killed her. The man who was treating her also said that there were things inside of her that was moving. And when he dumped her body in the cold water, the things came out and (he didnt say this but the description pretty mich fot the bill) i think the things that came out of her were small dragons that havent even completely formed yet. So, i believe that the valyrians used women as dragon incubators. (Now, im not saying that they always did that, im saying thatthat did happen.) And im thinking that blood magic was what did that right? So, we know that rhaenyra's daughter, visenya and dany's son, rhaego had the features of dragons. Scales and allat. So, you know what we also know? Those two did some kinda old valyrian tradition or some kinda blood magic. In dany's case, she did the blood magic thing first when she ate a horse's heart and second time when she wanted to bring drogo back.
Anyways, OKAY SO, the old valyrians used blood magic to secure the safe delivery and the survival of the both baby and the mother. And when they stopped doing the blood magic and converted to more westerosi faith (like, the seven, who hate blood magic) this was when the women of old valyria managed to barely survive child birth, or die in it. And same goes for the babes. The children either didnt live, were stillborn or got to live for a short period of time.
I think that this was because the 'blood of old valyria' stoppes doing what the old valyrians did. For example, blood magic and worshiping the fourteen flames
So. The survival of the children & the mother and the fertility of the women of old valyria was tied back to blood magic. And the most of the targs (man who couldn't have children. (Women who suffered with losing their children/died during childbirth) happened because they lost their connection to blood magic.
Blood magic was also why i think that the blood of old valyria managed to stay so...untouched by incestuous looking faces or disabilities.