After 13 weeks your baby can curl up and stretch out, so measuring the length becomes less accurate.
Then the circumference of the head becomes the best way to measure your baby Some hospitals and private clinics offer a dating and screening scan at the same time.
This is because blood tests for Down syndrome are not very accurate if you are having more than one baby.
Instead, you will be offered a nuchal translucency (NT) scan.
If your uterus (womb) is very deep in your pelvis, or if you're overweight, you may be offered a vaginal scan, which can get nearer to your baby.
It shouldn't be uncomfortable, and you don't need a full bladder.
The dating scan will take between five minutes and 10 minutes.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.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.If you have an irregular cycle, or if you've recently been taking the contraceptive pill, working out the date from your LMP isn't a very accurate method. The scan can check that your baby has a heartbeat and is developing normally.Your baby's head, limbs, hands and feet, and some organs can be seen.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.Nearly all scans after 10 weeks can be done through your tummy, and most units ask you to come with a full bladder.The sonographer will put some gel on your tummy and will move a small hand-held device (a transducer) over your skin to get views of your baby.