The ultra sound is pretty accurate, but it is not always correct on how far along you are in a pregnancy. Everything is an estimation when it comes to determining a due date. Babies tend to develop at close to the same rate, but every baby is different. It is a very educated estimate for how far along you are, but it is just an estimate.
- how far along in pregnancy you have to be to get an ultra sound? – http://answers.yahoo.com/question/index?qid=20080713061249AAObFD3