Most women will be advised to have at least two ultrasound scans during their pregnancy — the first and second trimester scans.
A first-trimester scan (usually between week 11 and week 14, also called a nuchal translucency test) confirms how many weeks pregnant the woman is and, therefore, the due date of the baby.
This scan can also help to determine whether the baby is developing inside the womb rather than outside it (a fertilised egg that implants and grows outside of the womb rather than inside the womb is called an ectopic pregnancy).
Along with a blood test, the first-trimester scan can be used as a screening test for Down syndrome, but it is not a conclusive test for this chromosome disorder.
A second-trimester scan (usually between weeks 18 and 20) checks for structural abnormalities, particularly in the baby's head or spine. The sex of the baby can also be determined at this time, if the parents want to know.
Third-trimester scans are sometimes recommended to women who have previously given birth to a baby with a low birth weight, who are carrying twins, or who have other medical complications.