Do dating scans date from conception
If you are having a screening test for Down syndrome, you'll be offered a dating scan first.
The test needs an accurate due date to be reliable, as hormones in your blood vary according to your stage of pregnancy.
You probably know when your LMP was, but not when you actually conceived.
If you do know the conception date, this is counted as day 14, and not day one.
You'll be given a report of the scan which will tell you exactly how many weeks pregnant you are.
Keep it with your maternity notes so your midwife and anyone else involved in your care can see it. This measurement, known as the crown rump length (CRL), is very accurate in the first trimester.
For most mums-to-be, the first scan will be a dating scan at between 10 weeks and 13 weeks plus six days of pregnancy.
However, the timing of your first scan will also depend on how your pregnancy is going, and where you live.
You may also be offered an early scan if you've had a previous miscarriage.
Having a scan in the first few months of pregnancy can be a thrilling experience.
It's amazing to see the little blob with a heartbeat that will soon become your baby.
You may have calculated the length of your pregnancy from the first day of your last menstrual period (LMP).
This is even though you don't actually conceive until 14 days after your LMP, or later than this if your cycle is longer than 28 days.