Obstetrics & Gynaecology

During a routine antenatal ultrasound, key assessments typically include foetal heart pulsation, foetal size, amniotic fluid volume, and the position of the placenta. In a first-trimester foetal morphology scan, additional structures are evaluated for early anomalies. These include the cranium, falx cerebri, choroid plexus, orbits, facial profile, nasal bone, mandible, nuchal translucency, spine, four-chamber view of the heart, foetal heart rhythm, lungs, stomach, umbilical cord insertion, long bones of the limbs, and the orientation of the hands and feet.

A first trimester foetal morphology scan is usually performed during 11–14 weeks of gestation. The more advanced the gestational age, the easier the foetal parts can be assessed.

A first trimester foetal morphology scan can pick up about half of the major foetal structural abnormalities. As the foetal size is small at this stage and the foetus will continue to develop, some major malformations may not be obvious at this stage or may develop in later pregnancy. Hence, not all major foetal abnormalities can be detected at this stage, even with the appropriate ultrasound equipment and in the most experienced hands. Sometimes, structural abnormalities might be suspected but not confirmed on a first trimester morphology scan, in which case a follow-up morphology scan will be needed a few weeks later. 

The accuracy of a morphology scan in general depends on a number of factors, including the severity and nature of the abnormality, whether the foetal position is optimal for scanning, maternal factors such as abdominal thickness or the presence of surgical scars, and the quality of the ultrasound machine. Also critical is whether the medical personnel have experience or training in morphology scans.

A first trimester morphology scan is regarded as an extra scan performed in the first trimester and not a substitute for a Down syndrome screening test or second trimester morphology scan (usually performed at 20-22 weeks of gestation). 

Some gross malformations may develop later in pregnancy and may not be detected in the first trimester. If you are concerned about the possibility of your baby having Down syndrome, we advise you to have a Down syndrome screening test and routine second trimester morphology scan even if the first trimester morphology scan is normal.