###Human: How accurate are ultrasounds in determinig the gestational age and conception? ###Assistant: Hello,  Welcome to icliniq.com. The first-trimester ultrasound scans done before 12 weeks are the most accurate ones in the assessment of gestational age with an error of only + /- 5 to 7 days in the assessment of gestational age. So they are used for obstetric dating and determination of expected date of delivery. Moreover, if you have done 6 ultrasounds and the date calculated from each of them corroborates and does not lag behind then probably your baby is growing well and it is not small as you apprehend. The date we calculate from the first day of last periods is accurate only if the periods were regular for the last 3 months and there was no history of miscarriage or ovulatory disorder like PCOS (polycystic ovary syndrome) or contraceptive pill intake. In case of any discrepancy the date from dating scan at 6 weeks are accurate and to be considered. It is a probability that you actually conceived in later part of the cycle which leads to this discrepancy between the date and USG.