The Humerns are quite an odd species when it comes to mating. They have nine distinct sexes (not genders, they’re biologically distinct) all of which exhibit varying levels of dimorphism and all of which can interbreed with any sex but their own. Don’t even ask about their social structure or mating rituals...
Any given coupling has a distinct set of probabilities associated with it for the sex of the offspring (similar to these guys). Some couples cannot have children of a certain sex, others are near guaranteed to have children of a certain sex. The complex social and physiological harmonics this sets up are an evolutionary response to a harsh and ever changing planetary ecology, and ensures that their species can adapt to rapidly changing climates and survival requirements without actually changing too much.
Each pair will have exactly two children, as breeding is done by both parents interlocking and flooding a temporarily shared ‘womb’ with their gametes before separation. The gametes combine, embed in the lining of the parent’s womb, establish a small, simple placenta, mature into a larval/embryonic stage then engage in a frenzied bout of siblicide that ensures only one child is brought to full term per parent.
The question is this:
By what method can many (potentially hundreds) of these embryos be reduced to exactly one while minimising the risk of death of the survivor and also minimising risk to the parent?