It won't hurt, although you may feel a little pressure on your tummy.
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.
Your baby's head, limbs, hands, and feet can be seen, as well as his abdominal wall and some organs, such as his stomach.
Finding out about having twins early in pregnancy also gives you more time to prepare for the birth and for your doctor and midwife to plan your care. Nearly all scans after 10 weeks can be done through your tummy, and most units ask you to arrive with a full bladder.
The sonographer will put some gel on your tummy and move a small hand-held device (transducer) over your skin to get views of your baby.
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.
The dating scan will also show if you're expecting twins, triplets, or more.