Seahorses reproduce in an unusual way. The male seahorse becomes pregnant instead of the female. Most seahorse pregnancies lasts approximately 2 to 3 weeks. The male seahorse has a brood pouch where he carries eggs deposited by the female.

The reproductive behaviour of seahorses is notable in that the male carries the fertilized eggs. After an elaborate courtship, the female uses an ovipositor (egg duct) to place her eggs into a brood pouch located at the base of the male’s tail where the eggs are later fertilized.

The reproductive process begins when a male and a female seahorse do daily pre-dawn dances, intertwining their tails and swimming together. Eventually they engage in a true courtship dance, which can last as long as eight hours. It ends with the female depositing her eggs in the male's pouch.

Male seahorses and their close relatives, pipefishes and sea dragons, are the only male animals in the whole animal kingdom that get pregnant and have babies. A female moves eggs into the closed brood pouch of a male. The male then fertilizes the eggs, so he knows for sure who the father is, which is unusual for an animal.

Instead of females, male seahorses carry the developing seahorse embryos in a kangaroo-like pouch. During mating season, the female deposits her eggs into the pouch, and the male fertilizes them.