By week 8 he can be sure to see something even if they woman has missed her guess about when she got pregnant.
I started ovulating October 22nd 29th 2015 i had sex October 25th 27th now i am pregnant i did a blood test the doctor gave me an estimated conceive/ conception date which was on or around November 6th and my estimated due date is or around August 1,2016 could that be right.
But for the purposes of dating pregnancy, the sweet spot is after the beginning of the 6th week and up until about the beginning of the 8th week.
They are not useful in the 4th or 5th week, the 4th because it is too early to see the embryo, and the 5th because a lot of times the woman has not ovulated when she thinks she did, and the baby is not showing up because it's too early to see it (in other words, what she thinks is her fifth week is actually her fourth).
They are less useful for dating the further along they go; I'm never too happy when someone says they had an "early" ultrasound and it turns out to have been in her tenth week, for example.
Some doctors tell women by week 12 that they could have a margin for error of /- 7 days already, and it doesn't just go from week 11 - margin for error 0, week 12 - margin for error, 7 days. However, you have said your ultrasounds have been measuring in a consistent manner, so it sounds like you have little likelihood of your baby growing faster or slower than average and throwing off the ultrasound measurements. All the best Mx Ultrasounds carried out between 8 and 11 are the most accurate for determaing conception scans at 6 or 7 weeks is too early that's why they call ur 12 week scan a dating scan they have a margin for error of 3-5 days when done between 8 and 11 weeks hope this helpa Hi, Febbaby272, ultrasounds carried out in the 6th or 7th week are the most accurate for determining conception, if they are not in error in terms of timing (which can happen if the woman got the ultrasound too early thinking that she was in her sixth or seventh week when she was not).
A doctor might logically ask a woman to come in later rather than earlier for an ultrasound if the woman has irregular cycles, because the doctor doesn't want to waste time on a scan that's too early.