If you prefer, you can have a private screening test.
As well as the combined test, most private healthcare clinics offer non-invasive prenatal testing (NIPT), which is the most accurate (up to 99 per cent) screening test for Down's Syndrome.
It may also help you to avoid feeling disappointed towards the end of your pregnancy if you think your baby is late when he's not.
The scan can check that your baby's heart is beating nicely and that he's developing normally.
For example, the NT scan for Down's syndrome has a false-positive rate of five per cent.
This means that, out of 20 women who have the scan, one woman will be wrongly told she has a high chance of having a baby with Down's syndrome.
It shouldn't be uncomfortable, and you don't need a full bladder Your sonographer will give you 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.
If your womb (uterus) is very deep in your pelvis, or if you're overweight, you may be offered a vaginal scan, which can get a closer view of your baby. The scan will usually show that your pregnancy is less far along than suggested by your LMP. This means that accurate dating reduces your chance of having your labour induced because your baby is thought to be overdue. It's entirely up to you whether or not you have a scan.
You could have a dating scan without NT screening, or even change your mind on the day of your scan. If you do know the conception date, this is counted as day 14, not day one.