The mating pair entwine their tails and the female aligns a long tube called an ovipositor with the males pouch. The eggs move through the tube into the males pouch where he then fertilizes them. The embryos develop in ten days to six weeks, depending on species and water conditions.

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.

When mating, the female seahorse deposits up to 1,500 eggs in the male's pouch. The male carries the eggs for 9 to 45 days until the seahorses emerge fully developed, but very small. The young are then released into the water, and the male often mates again within hours or days during the breeding season.

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.