Erdogan's Curse
Long after the various European wards are being rebuilt after Krakatoa, as Queen Victoria was being laid to rest. A publication flooded the various magi-journals and the Ministries outlining source and the effects of Erdogan's Curse upon European magery. And to explain why one in three or more magical female births in Europe are multiples with a quarter of these being identical twins and one in ten being triplets and why female births would outnumber male births thirteen to five by 1920s.
In America it is called Smiths curse as it has crossed over into the Real and was first noted among the Mormon religion, even though later records found that the curse was common among all the magical populations in the effected areas.
The curse was discovered by a Turkish ambassador to France in 1523, a man of patience and diligence who enjoyed history and figuring out why people had done things, but few paid attention to the ramblings of a foreigner. And many later blamed him for it and named the curse after him. Even though the source is recorded at Sabat and completely supports the late ambassadors work.
To be blunt the curse was cast by a dying Nimue, then one of Avalon's last remaining high priestesses of the pre-Christian religion. Originally it was only supposed to be target against the families of men who were Arthurs slayers and to avenge the death of her children at the hands of the rebel wizards. But there was a one last raid against the sanctuary she had chosen to conduct her curse ritual and a Saxon mercenary killed her at an inopportune time. So thus the curse spread to all of England and eventually Great Britain; three daughters for every son, the curse was slow to spread but to date has infected every national ward across Europe, Asia, North Africa and North America with strong indications of it having already spread to Australia and Micronesia within the last 50 years. And this has aggravated and changed magical society on the whole for 100 years, and Avalon for very nearly its whole existence.
Its spread was aggravated because of an outgrowth of having both fey/drakon blood and powerful exposures to the magical release of nodes, and to the magnifying effect that many forms of permanently enchanted wards. For Avalon it came about because of the various magical sanctuaries being built atop nodes and eddy's in the ether to construct and shield these havens.
It functions by making most women susceptible to hyper ovulation when exposed to high mana or thaum fields normal to warded properties and the dimensional pocket spaces and by delaying the implantation signal in a womb at the time of conception causing a quintupling of fraternal birth rates among a third to half of all girl births in the first days.
Because of Christian and later Purist influences the formation of Covens that would have been able to absorb the excess are not formed except in Avalonian holdings after Camlan and the Coven pattern of marriage will not spread to the wider magical world until the 1850s. Allowing the rise of truly horrible institutions like the slavers of the House of Chains and their backers across Europe. Breaking the House of Chains power in Europe will take nearly a hundred years and begin soon after the House of Chains is broken in North America following the American Civil War.