Most women can instead have their first scan at 11 to 13 weeks — this is usually called the 12-week scan. This scan can work out the due date and gestational age of the baby, if youre having more than one baby and also screen for conditions like Down syndrome.

How many days out can a dating scan be?

You can have the combined screening test from 11 weeks to 13 weeks plus six days, or when your babys CRL is 45mm (1.8in) to 84mm (3.3in). As long as the timings are right, you can have the NT scan and blood test at the same appointment as your dating scan.

How far off can an early ultrasound be?

Ultrasound dating of conception is not reliable for determining paternity because the test can be off by at least 5-7 days in early pregnancy.