A female mule that has estrus cycles and thus, in theory, could carry a fetus, is called a "molly" or "Molly mule", though the term is sometimes used to refer to female mules in general. Pregnancy is rare, but can occasionally occur naturally as well as through embryo transfer. A male mule is properly called a "horse mule", though often called a "john mule", which is the correct term for a gelded mule. A young male mule is called a "mule colt", and a young female is called a "mule filly".