Less of a magic item and more of an enchantment found in certain lubes, Viagra, potions, oil diffusers, bath salts, etc it can be occasionally found on a condom or other forms of birth control although this version is frowned upon. The "Mother Fucker" enchantment's function is to either enchant a man's cum so when it enter's a mother's womb the enchantment will activate or it'll enchant a mother's womb so when a man's cum enters it'll activate. At this point the enchantment will activate and one of the mother's children will change. Generally this change will effectively replace the chromosome given by their father, but certain enchantments can have additional or alternative effects. The Condom/Birth control for instance replaces the chromosome given by the father with nothing turning the child into an unfertilized egg, but there are variations of the enchantment.
It is recommended to use the "Mother Fucker" to help a first child with any genetic or potentially developmental issues. If used later on may be used to help with gender dysmorphia. It is less recommended to use the "Mother Fucker" when the mother has had multiple children. Certain enchantments may act oddly, choose a child randomly, or fail in unexpected ways depending on the enchantment.
Let is be known that the "Mother Fucker" enchantment has had other workers trying to create derivatives. These derivatives may be even less stable than the "Mother Fucker" line of enchantments. It is not recommended to use these derivatives so don't use them or use them cautiously if they are around. The "Mother Fucker" enchantments are not responsible for side-effects including- and that's where most people start tuning out as the magic mouth starts speeding up to cram so many niche magical effects that could happen.