Spontaneous and experimentally induced twinning and conjoined placentation were studied in Swiss‐Webster (SW) mice. Conjoined placentation was noticed at a rate of 0.4% for twins and 0.06% for triplets. Placentally conjoined embryos were either symmetrical or asymmetrical. A single spontaneous monozygotic conjoined twin was observed among 2,000 blastocysts cultured in vitro. The yield of twins could not be increased by manipulating explanted embryos in vitro, or by treating in utero during the early postimplantation stage of development with vincristine.