Typically we mamas get the news that we're having twins lying on the table during our first ultrasound.  Some people may have other signs where the can guess that there are more than one...like increased symptoms (some don't have any at all), bigger in size than normal for first trimester (however many women grow and carry uniquely), etc.  Some may at first learn about it at an early (pre-ultrasound) family doc/midwife/OB appointment where two heartbeats are heard using the doppler (some may not pick it up that soon though).  Regardless of when you find out, when you officially find out from one of your health care providers, you are likely to bombarded by a barrage of information...mostly likely starting with what type of twins you're having...

I've come to understand this better by likening it to types of dwellings the babies are in during pregnancy.