Popular folklore does not stipulate one particular method of mermaid reproduction. Folklore depicts mermaids as creatures who live in the sea and have the upper body of a woman and the lower body of a fish. To explain the existence of multiple mermaids, there are numerous theories as to how mermaids get pregnant.
There are many paintings depicting mermaids in different forms. Some show mermaids with forked tails, suggesting that the forked tails act as reproductive organs for the mermaid. Some artistic works depict the mermaid as human on the bottom and fish on the top, or as a sea creature that can switch from mermaid to human when enticing sailors to their deaths.
Popular fan fiction explores the possibility that a mermaid lays eggs like other fish species, and the mermen fertilize the eggs. Since mermaids are mythological creatures, how one becomes pregnant is left open to interpretation and imagination.