well you did ask
The mermaid secretes a fine mucus containing millions of small eggs into the water. The eggs are ingested by a host, in this case human, and hatch within the gut into billions of small worm like creatures. If the host is male the worms slowly, over many years, consume the host from the inside until all is left is a writhing corpse. If on the other-hand the host is female the miracle of life continues as the worms travel into the womb. Any existing host fetus is consumed. The worms then bind to the womb and hijack the fertilization process of the host, injecting their own genetic material into the process. Any fetus developing in the host's womb will now have "three parents". It takes 4-6 months for the chimera fetus to grow into an infant mermaid, a hybrid of mermaid and host parents, at which point it sits in wait for the host to come into proximity with water. Once that happens the female host will give spontaneous birth to a slimy eel-like creature. The infant's instinct is to flee to deep water. Over subsequent years the infant will grow and features and characteristics of the host will emerge. In the case of a human host the resulting mermaid will appear half-human. The reason why most mermaids are typically human female is because the fertilization hijacking process breaks the typical 50/50 male/female stat and instead biases for 95/5 female/male.