Male pregnancy is the incubation of one or more embryos or fetuses by organisms of the male sex in some species. Most species that reproduce by sexual reproduction are heterogamous—females producing larger gametes (ova) and males producing smaller gametes (sperm). In nearly all animal species, offspring are carried by the female until birth, but in fish of the family Syngnathidae, males perform that function.

The Syngnathidae is a family of fish which includes seahorses, pipefishes, and seadragons. The name is derived from Ancient Greek: σύν, meaning "together", and γνάθος, meaning "jaw". The fused jaw is one of the traits that the entire family have in common.