Ultrasound Issues

Cbutler07

At 20 wks I had my first Ultrasound and I was told that I was having a girl. The Dr. printed a picture and circled the 3 lines representing the hamburger as they call it. Well I am 28 wks and Tuesday went back to make sure my placenta has moved and to our suprise another Dr. said it was a boy. Showed us a picture of his bum and testies. (I should mention that I go to a place that has 2 doctors that do ultrasounds and 3 that does delievery) I am still kind of in shock and DH and I have decieded to go get a 4-D ultrasound just to confirm the sex,since everything we have thus far is for a girl. Anyone else have this happen? What  can I expect differently between a regular ultrasound and a 4D?