The 1st trimester transvaginal USscan – fluid sac with nothing in it = 5 wks 1st trimester transvaginal USscan – fluid sac +yolk sac (no CRL) = 5.5 wks 1st trimester transvaginal USscan – fluid sac +yolk sac+fetal heart + CRL 0-4mm = 6 wks In the remaining 1st trimester in the fetal pole can be identified then do CRL 2nd and 3rd trimester do BPD, OFD femur length and abdominal circumference