How and when are mulitple gestation pregnancies diagnosed?

Well ideally, multiple pregnancies are diagnosed early on in the pregnancy, during the first trimester by ultrasound. In the past we used to diagnose these by clinical methods, when a mom was measuring bigger than the expected gestational age. But that's no longer sufficient. We now know that there's multiple variations in the number of placentas and the number of gestational sacs around the baby. And the only time you could accurately diagnose that is in the first trimester, preferably before 12 or 13 weeks gestation.