How off can an ultrasound be that estimated your last menstrual period the same at eight weeks and twelve weeks?

Answer 1

I think this is what you mean. For example. On 28th December the ultrasound said you were 8 weeks pregnant, then on 25th January it said you were 12 weeks pregnant. This would be accurate to within 4-5 days each time.

Answer 2

Aftere having had two healthy babies and one miscarriage, I have found ulrasounds to be extremely accurate in dating a pregnancy. My OB is now only performing the initial ultrasound at 8 weeks due to the fact that so many pregnancies end in spontaneous miscarriage (20% of all pregnancies). They would not have patients wait until 8 weeks if they felt the process would inaccurately date the pregnancy. Dating is important in order to make sure that screenings are done at the appropriate times for accurate results (quad screen, glucose test, group b strep swab, etc.). I hope this helps you and good luck!