Depending on your stage of pregnancy, scans can:
Check that your baby has a heartbeat.
Say whether you're pregnant with one baby or twins or more.
Detect an ectopic pregnancy, where the embryo implants outside the womb, usually in the fallopian tube.
Find out the cause of any bleeding you may be having.
Accurately date your pregnancy by measuring your baby.
Assess your baby's risk of Down's syndrome by measuring fluid at the back of your baby’s neck between about 10 weeks and 14 weeks (nuchal translucency (NT) scan).
Find out why a blood screening test was abnormal.
Help with diagnostic tests, such as chorionic villus sampling (CVS) or amniocentesis, by showing the position of the baby and placenta.
Examine your baby to see if all his organs are normal.
Diagnose most abnormalities, such as spina bifida.
Assess the amount of amniotic fluid you have and find out where the placenta lies.
Measure your baby's rate of growth over several scans.
A scan may show if you are expecting a girl or a boy. But if your baby's lying in an awkward position, it's not always easy to tell. Plus, some hospitals have a policy of not revealing the gender of a baby.

18week Prenatal ultrasound