Dating pregnancy with ultrasound
You may need to have a full bladder for this scan, as this makes the ultrasound image clearer.
How big your baby will be depends on when you have the scan: After 13 weeks, your baby can curl up and stretch out, so measuring the length becomes less accurate.
You may know when your LMP was, but not when you actually conceived.
If you do know the conception date, this is counted as day 14, not day one.
If you have agreed to have screening for Down's syndrome, the dating scan and the screening will usually happen at the same time.
Find out more about the combined screening test for Down's syndrome.
If your cycle is irregular or you've recently been on the pill, working out the date from your LMP probably won't work. If the scan date differs from your period dates, the scan date will be used.