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Irish Maternal Fetal Foundation

Suite 36, Beacon Hall,
Beacon Court,
Sandyford, Dublin 18
01 293 3984
01 293 3971
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Fetal Anomaly Scan (FAS)

Most women are offered a detailed ultrasound scan at about 20 weeks to see if your baby is developing normally. This is known as the Fetal Anomaly Scan (FAS).

Why have a Fetal Anomaly Scan?

The vast majority of babies are normal. However all women, whatever their age, have a small chance of delivering a baby with a physical or a mental problem. Many such abnormalities can be diagnosed and ruled out with the Fetal Anomaly Scan. Reasons to have this scan include:

  • To reassure you that your baby is likely to be normal
  • To confirm the gestational age of your pregnancy
  • To confirm the number of fetuses and, if twins, whether they are identical or not
  • To detect birth defects, such as a spina bifida or heart problems
  • If you are concerned about the chances of chromosome problems like Down syndrome, this scan can search for subtle markers that may suggest a higher risk that your baby may have one of these problems
  • If you want to know your baby's gender this can usually be seen at this scan

When you attend for this scan we will tell you about everything that we see, unless you advise us that there are certain things that you don't want to know about, such as your baby's gender or markers for chromosome problems.

What can this scan detect?

This ultrasound scan is very accurate but unfortunately it cannot diagnose 100% of congenital abnormalities. If the scan is complete, we would expect to pick up at least 95% of cases of spina bifida, 80% of cases of cleft lip or palate, and 60% to 70% of cases of congenital heart disease.

This scan can also identify 50% to 70% of cases of Down syndrome, but the First Trimester Screening (FTS) test is better for this. Because 30% to 50% of cases of Down syndrome appear normal on ultrasound, only an amniocentesis can give you this information for certain. It is also important to realise that ultrasound scans in pregnancy do not detect problems like cerebral palsy or autism.

Sometimes babies with chromosomal abnormalities have signs called ultrasound markers. These include thick skin in the neck, excess fluid in the kidneys, short arms or legs, white spots in the baby's heart or abdomen, or choroid plexus cysts in the brain. While some babies with chromosomal abnormalities have these markers, it is important to remember that many normal babies also have these signs.

The only way to diagnose or exclude a chromosomal problem for certain is to have an amniocentesis. If you would prefer not to know about these markers please inform us prior to the scan. If the scan suggests a problem, you will be told this immediately. You will be able to discuss the findings immediately with a consultant who specialises in fetal medicine.

Patient Information about FAS
Explaining FAS

A full support service will be available for you should any problems be detected, including a referral to an appropriate pediatrician. A copy of your report will be sent to your referring hospital, doctor or midwife to ensure good communication.