Obstetric Anomaly Scan
The obstetric anomaly scan (mid-trimester or 18 to 22 week scan) is a detailed transabdominal ultrasound examination of the foetus, placenta, amniotic fluid and cervix. It is done at 18 to 22 weeks (most often 20 to 21 weeks). It is used to detect structural anomalies, assess the placental site and the cord, measure amniotic fluid, confirm the number of foetuses and check growth against dates.
A 3 to 5 MHz curvilinear probe is used, with a fixed set of views:
- Head (biparietal diameter, head circumference, lateral ventricles at 10 mm or less, cerebellum, cisterna magna, cavum septi pellucidi, choroid plexus).
- Face (profile, orbits, lips and nose).
- Spine (longitudinal and transverse views to check closure of the vertebral arches and skin cover).
- Thorax (lungs and diaphragm).
- Heart (four chamber view, left and right outflow tracts, three vessel and tracheal views).
- Abdomen (stomach bubble, kidneys, bladder, bowel, cord insertion, abdominal wall).
- Limbs (long bones, hands and feet).
- Genitalia when requested.
- Placenta.
- Cord (three vessels).
- Amniotic fluid.
- Cervical length by transvaginal scan where indicated.
Measurements include abdominal circumference and femur length for biometry. The scan takes 20 to 45 minutes. Three dimensional ultrasound may add surface detail, and colour Doppler adds vessels.
Major structural anomalies occur in 2 to 3% of births.
The mid-trimester scan detects about 50 to 80% of major anomalies in routine practice. Detection varies by organ:
- Neural tube defects and anencephaly over 90%.
- Major cardiac defects 40 to 60% with a four chamber view and up to 80% with outflow tracts.
- Renal anomalies 70 to 90%.
- Abdominal wall defects over 90%.
- Skeletal dysplasias 40 to 60%.
- Cleft lip 60 to 90% and cleft palate under 30%.
Detection allows counselling, further testing, planning of delivery in a centre with surgical and neonatal services, in utero treatment in selected conditions, and options for continuing or ending the pregnancy where the law permits.
Indications. Routine mid-trimester screening for all pregnancies.
Detailed scan for increased risk: previous child or family history of anomaly, maternal diabetes, epilepsy treatment, infection exposure, teratogen exposure, abnormal serum screening, raised nuchal translucency, assisted conception, multiple pregnancy, abnormal foetal findings at an earlier scan, polyhydramnios or oligohydramnios, and growth abnormalities. Placental localisation.
Assessment of cervical length in women at risk of preterm birth. Foetal sex determination only where it is clinically relevant (sex linked disorders). Not for sex selection or non-medical purposes.
Contraindications and precautions. None. Safe at normal diagnostic output (thermal index under 0.7 for first trimester examination of the embryo, mechanical index under 1.0), with exposure time as short as the examination requires. A full bladder is not needed after the first trimester (and may be unhelpful).
The woman should be informed before the scan about the purpose, the limits of detection, and the possibility of finding an anomaly, and consent is obtained. Counselling and support are available for abnormal findings. Transvaginal scanning is used with consent and a chaperone when a low placenta or short cervix is suspected.
Preparation and technique. The woman lies semi-recumbent, tilted slightly to the left after 28 weeks to avoid aortocaval compression. Gel is applied and a systematic sequence is followed. If views are limited by foetal position, maternal habitus or movement, the woman may be asked to walk, empty her bladder, or return for a repeat scan after 1 to 2 weeks. The report states the organs seen and the organs not seen (incomplete examination).
