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. It won't hurt, although you may feel a little pressure on your tummy.
Ultrasound scans are considered to be safe for you and your baby when performed by trained professionals.
The dating scan isn't designed to look for more subtle abnormalities, though.
That will happen at your next routine scan at about 20 weeks However, some major problems can be seen at your dating scan, such as issues with the umbilical cord or with your baby's skull or abdominal wall.
Hormones in your blood vary according to your stage of pregnancy.
A blood test at what you thought was 13 weeks wouldn't be helpful if you were actually 16 weeks pregnant.