They summoned the goddess Yogamaya and had her transfer the fetus within the womb of Devaki for the womb of Rohini (Vasudeva's other spouse who lived along with her sister Yashoda throughout the river Yamuna, from the village of cowherds at Gokulam). Hence the kid conceived in a single womb https://digitalasset.us-lax-1.linodeobjects.com/SouthernCaliforniaSurrogacy/Becoming-A-Surrogate-In-Southern-California.html