Pregnancy Dating Discrepancy: Understanding Ultrasound vs. LMP

Question
My daughter is pregnant. She has irregular periods. Some time she has 2 in 1 month some times she has 45 day cycles. According to her lmp she is 12 weeks pregnant according to ultra sound she is 8 weeks pregnant. She knows that she conceived dec 4. So what date will the Dr. go by? The lmp date is almost 1 month off from the others.  

Answer
The doctor will most likely use the ultrasound to date the pregnancy, esp. since they are very accurate early on (12 weeks or before). The reason early ultrasound is considered more accurate is it can actually take measurements of the baby and more closely pinpoint what the gestational age is.

Regards,
Ann S