Fetal Anomaly Scan

Fetal Anomaly Scan (Detailed scan)

Fetal Anomaly Scan is also known as detailed scan, 20 weeks scan or mid-pregnancy scan.

The aim of the scan is to look for baby's anatomy in more details, try to find out developmental and structural abnormalities of the baby, and to determine the position of placenta. The scan will detect 80% of babies who have serious physical abnormalities, for examples, spina bifida or hole in the heart. The best time to do this scan is at 18 to 20 weeks of pregnancy. Because the baby's parts are not crowded and easily seen. The scan takes about 20 minutes.

What are examined in a fetal anomaly scan?

The scan will show fetal anatomy including lips, nose, legs, feet, heart, lungs, stomach, kidneys, bladders, umbilical cord and placenta. Major abnormalities in these parts will be detected during fetal anomaly scan. The liquor volume will be assessed at the same time. Excessive or decreased of liquor volume is always associated with fetal anomaly. During scanning, the position of the placenta will be identified.

What are the abnormalities that can be detected?

About 20% of all abnormalities will not be seen in the fetal anomaly scan because these abnormalities are difficult to detect or they appear at the later part of your pregnancy. So, there is a small chance that your baby may have abnormalities at birth which are not detected during fetal anomaly scan.
The table below shows the common abnormalities that can be detected and the chances of being seen in a fetal anomaly scan.

Problem	What the problem is	Chance of being seen
Spina bifida	Open spinal cord	90%
Anencephaly	Absence of the top of the head	99%
Hydrocephalus	*Excess fluid within the brain	60%
Major congenital heart problems		25%
Diaphragmatic hernia	A defect in the muscle which separates the chest and abdomen	60%
Exomphalos/gastroschisis	Defects of the abdominal wall	90%
Major kidney problems	Missing or abnormal kidneys	85%
Major limb abnormalities	Missing bones or very short limbs	90%
Cerebral palsy	Spasticity	Never seen
Autism		Never seen
Down syndrome	May be associated with heart and bowel problems	About 40%

Source: Royal College of Obstetricians and Gynaecologists (2000)

What if my scan is abnormal?

Most of the time, the scan will not review any abnormality. In fact, 98% of the babies born are normal. If an abnormality is detected, you will be rescanned by another doctor to confirm the diagnosis. If the problem is serious, example anencephaly, and the baby cannot survive after birth, then termination is the option. If the problem requires surgery soon after birth, example gastroschitis, then you will be transferred to a hospital that can perform the surgery soon after birth. If the abnormality is associated with chromosomal abnormality, then cordocentesis will be performed to determine the karyotype.

2D images of normal fetal parts

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