Adults are dioecious and live in copula within the vasculature of mammals. Each species has a preferred site. Of the main species infecting humans, S. haematobium lives in the vessels of the urinary bladder, S. mansoni in the portal veins of the large intestine, and S. japonicum in the veins of the small intestine. Gravid females leave the male and migrate to the terminal vessels, where they deposit unembryonated eggs. Eggs pass through host tissues, aided by the host's inflammatory response, and are released into the bladder or intestine, and voided with feces or urine. By the time eggs are released, they have embryonated and will hatch soon after entering water.
A miracidium hatches from the egg and must find a suitable snail host within a few hours or it will die. The miracidium apparently uses chemotaxis to locate snails, but the precise mechanism has not been identified. Each species can infect only a narrow range of snail species. For example, S. mansoni prefers Biomphalaria spp., S. haematobium prefers Bulinus spp., and S. japonicum prefers Oncomelania spp. After penetrating the intermediate host, the miracidium develops into a mother sporocyst. The mother sporocyst produces daughter sporocysts after a few weeks, and the daughter sporocysts begin producing cercariae 2-3 weeks after that. The sporocysts cause a chemically-mediated castration of the host snail; it no longer is capable of producing eggs but its survival is not affected. The sporocysts use the increased availability of food from the host to produce cercariae, and cercarial production continues for many months.
The cercariae also have a short life-span, and are infective only for about 1 day. To maximize the chances of contacting a definitive host, they usually emerge from the snail on a daily rhythm that coincides with periods of activity of local definitive host populations. The cercariae then alternately swim to the surface and sink slowly, maintaining their position in the water column. If they contact the skin of a suitable host, they penetrate it. Once in the host the tail-less cercaria, now transformed into a schistosomule, migrates through the tissues until it enters the circulatory system, which eventually transports it to its preferred site. There, maturation and mating occur. The adults can live for many years.