Missed Twin in First Tri US?

My first u/s (vaginal) was done when we thought I was 9w2d based on my LMP. Based on u/s measurement, I was really only 7w5d; my midwife found only one heart beating baby. I asked her to double check as my brother and sister are Fraternal Twins and my mom didn't find out until her 2nd Tri u/s AND my mother's brother and sister are also Fraternal Twins.
My midwife checked quickly and we both could only see one healthy baby. However, I am LETHARGIC this first trimester, like beyond belief, but no major morning sickness. Unlike my first pregnancy with my 4 yo DD; I was full of energy all first Tri but puking my life away! And this second time around, I am HUGE! I've always been on the thicker side but didn't really POP with DD until I was already 7 months along. Today, I am 9w2d and look 6 months pregnant! I know after your first pregnancy, the uterus is already stretched so you show much faster but really, my family and friends who do know about this pregnancy and have seen me with my first pregnancy, are asking if I'm having TWINS! I have my screening appt at 13 weeks so I am sure we will know for sure by then. 
But has ANYONE missed a twin in their first Tri u/s and if so, when did you find out?