Twin Pregnancy: Understanding Early Ultrasound & Fetal Heartbeats

Question
I am 28. I have 3 children.  I am due the middle of may.  My question is that I had an sonogram at about 6wks and couple days.  It showed what they said was only one child and one heart beat.  Could it be possible that being that far along they may still not be able to have seen a second baby?  I have a fetal heart beat monitor.  I am now 17wks and couple days.  When I listen to the heart beat I can pick up a heart beat on my left side and slide the monitor over and hear no heart beat in the middle and then pick up another heart beat on the right side.  I guess my question is how far can you move the monitor away and not hear that same heart beat.  I weighed around 125lbs before pregnant and now weigh around 148.  My belly is quite big and my belly button is already poking out on the top.  My hight is 5'4 so I was not over weight.  

Answer
If the sonogram that was done at 6 weeks gestation showed only one baby, I really doubt that you are carrying twins. You should have another sonogram at around 20 weeks and then you will know for sure. You can hear the heartbeat of a single fetus anywhere on the abdomen that the doppler monitor is placed. The only way to know if there is more than one baby is if you hear two different rates of heartbeats when listening at the same time at  different areas. Your weight has nothing to do with whether you are carrying twins or one baby. A sonogram is your best bet to know what you are carrying.