A heifer is generally a freemartin if the other twin is a bull. There are exceptions and tests to determine her fertility (but personally, I wouldn't retain one). Twin heifers & twin bulls are fine.I heard that twin calves are usually sterile. Then I heard only if they re split twins - m/f. So if a cow has twin heifers or twin bulls those calves will be fertile? Or was the whole "sterile twin" thing untrue? Just asking.