Can an Anomaly Scan Detect All Birth Defects?

A normal anomaly scan feels like a finish line. The report says "no abnormality detected" and families understandably relax. But an anomaly scan is a screening ultrasound, and no ultrasound, however skilled the examiner, can find every birth defect. Knowing where its limits lie helps you read the report for what it is.

Why does the scan miss some conditions?

Ultrasound shows anatomy. For the scan to pick up a condition, it has to change how something looks, and the change has to be large enough to see at that stage of pregnancy.

Conditions that do not change anatomy

Many genetic and chromosomal conditions, metabolic disorders, hearing loss and developmental conditions leave the baby's organs looking entirely normal. Some chromosomal conditions come with subtle ultrasound clues, yet plenty of affected babies show none.

Conditions that develop later

The scan is a snapshot taken at around 20 weeks. Certain brain, kidney and bowel findings, along with growth problems, only become visible as the baby grows. A clear scan at 20 weeks says nothing about what may show at 32.

Small or well-hidden findings

A small hole between the heart chambers can slip past even a careful examiner. The same goes for an isolated cleft palate, which sits behind the lip and is difficult to see from outside.

What affects the quality of the scan?

No two scans are equally easy. A baby lying with its back to the probe can hide the spine, face or heart. The amount of amniotic fluid, the mother's body build, the gestational age, the equipment and the examiner's training all influence what can be seen.

Sometimes the report will say that certain views were limited, and a repeat appointment is offered. Parents often read this as bad news. In most cases it simply means the images were not clear enough yet, and the baby needs a different position or a few more weeks.

What does a normal report actually tell you?

It tells you that no structural problem was seen in the views obtained on that day. That is a reassuring result and worth feeling relieved about.

What it cannot tell you is whether the baby has a genetic condition, whether something will appear later, or whether a functional problem exists that ultrasound is not designed to show. This is why obstetricians avoid saying "everything is normal" and prefer "nothing abnormal was seen."

When are further tests considered?

Most pregnancies do not need anything beyond routine scans. When there is a reason, the next step depends on the question being asked.

A fetal echocardiogram looks at the heart far more closely than the anomaly scan and may be advised after a family history of heart disease, maternal diabetes or an unclear heart view. Fetal neurosonography examines the brain in detail when a finding is suspected. Growth scans with Doppler follow the baby's size and blood flow in the later weeks.

Genetic tests answer a different question again. NIPT is a screening test that estimates the likelihood of certain chromosomal conditions, whereas amniocentesis and CVS are diagnostic procedures that can confirm them. Some conditions are recognised only when the newborn is examined after birth.

Questions worth asking at your scan

You do not need medical training to get more from the appointment. Consider asking:

• Were all the standard views obtained, or were some limited?
• Is there anything that needs a follow-up scan?
• Do my history or earlier test results call for any additional assessment?

When should you see a fetal medicine specialist?

Your obstetrician may suggest referral if you have had a previous baby with a congenital condition, if a genetic disorder runs in your family, or if you are carrying twins or have diabetes. An abnormal screening result, or a scan finding that needs a closer look, is another common reason.

Whatever your scan showed, contact your obstetrician promptly if you notice reduced baby movements, vaginal bleeding or leaking fluid.

Before your appointment

A scan report answers a specific question about structure at a specific moment. If something in your report or family history is on your mind, take it to your obstetrician or a fetal medicine specialist. Patients seeking fetal medicine consultation in Trichy can contact Trichy Fetal Medicine and Fertility Centre through the official website.

If this applies to your pregnancy, our anomaly scan page explains what the visit involves, or you can meet our specialists before you decide anything.

Related guides

• What Does an Anomaly Scan Check in the Baby?
• What Happens If an Anomaly Is Found During the 20-Week Scan?
• Fetal Echo vs Anomaly Scan: What's the Difference?