Also, the Y chromosome has slightly lower mass, and so the male sperm swim slightly faster. I would suspect, though I haven't seen any studies, that some ferti

Also, the Y chromosome has slightly lower mass, and so the male sperm swim slightly faster. I would suspect, though I haven't seen any studies, that some fertility treatment steps unintentionally have mass-sorting properties similar to the barrier diffusion Uranium enrichment technique used in the Manhattan Project.