The phrase "most knocked-up" is colloquial slang used to describe someone who is pregnant or has fathered multiple children. When looking for synonyms one might use in a more formal setting, a few options come to mind. "Expectant" or "pregnant" would be good alternatives when referring to a woman who is carrying a child. If referring to a man who has fathered multiple children, one might use "prolific" or simply say he has "fathered multiple children." More colloquial alternatives could include "preggers," "with child," "knocked-up," or "having a bun in the oven." Ultimately, the choice of synonym would depend on the context and tone desired.