In the ancient world, maternity death rates were very high due to lack of medical knowledge and understanding, as well as life-saving technologies. As such, it was a toss up to whether the woman and baby would both survive the process. In this world, maternal death rates are almost unheard of, with all the risk of dying being transferred to the child and sparing the mother. However, the number of males in a population has been on a decline in recent decades. This is because of the rapid increase of male infants being born dead. The resulting gender imbalance as led to females far exceeding males within society, which has caused a shift in the power dynamic within the Empire and has forced it to rethink many of its internal structures for how society is run. Marriages would occur between sister-wives, who would share a man between them. This form of marriage would take the form of a collection of woman-woman unions, with the male serving as the legal partner. These unions between females wouldn't be sexual, but a pragmatic and stable way to produce heirs. These unions would take place between two or more women, but be limited to one husband to prevent conflict between families over reproductive rights.
Due to a lack of biological knowledge, this has been rationalized in terms of the accepted religion. A man's strength and fortitude is tested even before birth, and only the strongest of will survive the process and earn the right to be born into the world. However, the reality is that a mysterious genetic disease has spread to all corners of the world, and has been carried down through the female line. This disease has left girl infants unscathed and unaffected, but has led to the decline in the male population. The disease has stabilized at the present time, infant males have a 50% percent chance of being stillborn.
How could a disease like this work where it would only affect male infants but leave female infants unharmed?