In early pregnancy, your due date can be predicted using the first day of your last period (LMP) by adding 280 days. However, once you have a 12 week (dating) scan then the date is determined from that scan onwards.
How accurate are 12 week ultrasound due dates?
The earlier the ultrasound is done, the more accurate it is at estimating the babys due date. Ultrasounds performed during the first 12 weeks of pregnancy are generally within 3 - 5 days of accuracy. The most accurate time is between 8 and 11 weeks gestation.
Does your due date change after an ultrasound?
Your doctor probably wont change your due date unless its significantly different from your ultrasound date. If you have more ultrasounds, each ultrasound report will contain a new due date based on the most recent measurements.
Is your 12 week scan from conception or last period?
Why would I have a dating scan? A dating scan might be recommended before 12 weeks to confirm your due date if you are unsure of your last menstrual period or your date of conception. The scan is optional and not everyone will have it. You can discuss whether you want or need a dating scan with your doctor or midwife.