Ultrasound scans can detect a range of abnormalities in a baby's development or anatomy. Some can be identified early in pregnancy, while others may not become apparent until later.

Some major structural abnormalities can be detected during the 12–14-week scan. By 19–20 weeks, the baby has developed further, allowing a more detailed examination of its organs.

Abnormalities that may be detected include:

• Brain and central nervous system abnormalities, such as differences in the development of the brain or spine.

• Congenital heart defects, which vary in severity and may require a specialist fetal heart scan.

• Abdominal wall defects, where some of the abdominal organs develop outside the abdominal wall.

• Kidney and urinary tract abnormalities, which may affect how the kidneys or urinary system function.

• Limb and skeletal abnormalities, such as clubfoot or abnormal bone growth.

• Cleft lip, which may occur on its own or alongside other abnormalities.

• Fetal hydrops (hydrops fetalis), an abnormal accumulation of fluid in the baby's body.

The severity of these conditions varies. Some require no specific treatment, others may need treatment after birth, and some can have a significant impact on the baby's health or chances of survival.

Can an ultrasound scan detect chromosomal conditions?

An ultrasound scan may sometimes identify features that suggest a chromosomal condition, such as abnormalities in the baby's development or anatomy. However, an ultrasound scan alone cannot confirm a chromosomal diagnosis.

Non-invasive prenatal testing (NIPT) assesses the chance of certain chromosomal conditions. If indicated, further tests such as chorionic villus sampling (CVS) or amniocentesis may be offered.

When is a fetal echocardiogram needed?

Fetal echocardiography is a specialist ultrasound examination of the baby's heart, performed by a paediatric cardiologist.

It may be recommended if:

• A possible heart defect is identified during a routine ultrasound scan.

• One of the parents or a previous child has been diagnosed with a congenital heart defect.

• Certain genetic conditions or other risk factors are present.

• An abnormal fetal heart rhythm is detected.

A doctor or midwife will assess whether the examination is needed and arrange a referral where appropriate.

What happens if the placenta is low-lying?

The position of the placenta is checked during the 19–20-week ultrasound scan.

Sometimes the placenta lies low in the uterus, close to the cervix. In many cases, it moves further away from the cervix as the uterus grows.

If a low-lying placenta is identified, a follow-up ultrasound scan may be needed later in pregnancy to check its position.

When are additional fetal growth scans needed?

Additional ultrasound scans may sometimes be needed to monitor the baby's growth and wellbeing.

This may be the case if:

• The baby appears to be growing more slowly or more quickly than expected.

• There is too much or too little amniotic fluid.

• The pregnant woman has diabetes or certain other medical conditions.

• The pregnancy involves twins or more babies.

These scans may include measurements of the baby's size, assessment of amniotic fluid levels and Doppler ultrasound to assess blood flow in specific blood vessels.

Ultrasound scans in multiple pregnancies

Pregnancies involving twins or more babies may require more frequent ultrasound scans.

Early in pregnancy, ultrasound is used to determine whether the babies share a placenta and whether they have separate amniotic sacs. This information helps determine the appropriate monitoring throughout the pregnancy.

The frequency of scans depends on the type of multiple pregnancy and whether any additional risk factors are present.

3D ultrasound scans

Standard ultrasound scans use two-dimensional (2D) images to examine the baby and assess its development.

In certain cases, three-dimensional (3D) ultrasound may provide additional information, for example when a closer examination of suspected abnormalities of the face or other parts of the body is needed.

3D ultrasound is not part of routine pregnancy scans and is mainly used when there is a specific clinical reason.