Yes in General they are sterile. There is that rare exception once in a great while. But they can carry a foal. How you ask? Through embryo transfers. As long as the Molly cycles (comes in & out of heat) they can be synced up with a mare and an embryo transfer can be preformed at day 7. They flush the embryo out of a mare and transfer it to a Molly. The molly is then put on progesterone for several months and is checked at day 14 for pregnancy. Once she is confirmed pregnant some are kept on progesterone for several months before taken off. They foal out as normal and can nurse the foal through weaning time just like a mare. Studies have shown this is successful in Molly's who cycle, But they have had no luck in Molly's that don't cycle.

Hope this helps.