The Mamu Mamu is the yukkuri equivalent of a vagina/uterus. It acts both as an orifice to accept another yukkuri's Peni Peni, receive the inseminating bean paste, and act as a uterus for developing offspring. Offspring who develop within the Mamu Mamu tend to be larger and more developed when born, though their birth carries greater risk to the parent and child. For further information about Mamu Mamu vs. Stalk births, see Reproduction.
The actual mechanics of a Mamu Mamu pregnancy is disputed; depictions range from the offspring simply developing within it, to offspring developing on a stalk, but within the Mamu Mamu instead of exposed outside. There is at least one depiction suggesting that they develop within "eggs" contained within the Mamu Mamu. Once the offspring is ready to be born, they are pushed out of the Mamu Mamu by the parent (possibly using the same mechanism which expands and inverts it into a Peni Peni).
Originally, Mamu Mamu pregnancies were depicted as resulting in a single offspring, though later depictions have two or three offspring; in any case, Mamu Mamu pregnancies results in fewer offspring than stalk pregnancies.