O&G · Antenatal care — fetal anomaly screening
Fetal structural anomalies and the mid-trimester anatomy scan
Also known as 18 to 20 week anatomy scan · Mid-trimester fetal scan · Fetal anomaly scan · Morphology scan
Exam-exhaustive FRANZCOG fellowship reference on fetal structural anomalies and the mid-trimester anatomy scan — the ISUOG systematic protocol, the cardiac views (four-chamber, outflow tracts, three-vessel-trachea), common anomalies (cardiac, neural tube, renal, skeletal) and their management, the soft markers with Agathokleous likelihood ratios, and the referral pathway. Anchored on the ISUOG 2022 mid-trimester guideline and the Carvalho 2013 fetal-heart guideline, globally tagged to MRCOG and ABOG.
Practise this topic
On this page
Study tools
Your progress
Saved on this device.
Target exams
Red flags
- A four-chamber view alone reported as a normal heart — outflow-tract and three-vessel-trachea views are needed
- An isolated soft marker pursued with invasive testing in a woman with low-risk cfDNA — over-investigation
- Hyperechogenic bowel dismissed as 'just a marker' — it carries a broad differential (aneuploidy, cystic fibrosis, infection, obstruction)
- A normal scan quoted as a guarantee — detection is high but never 100%
- Mild renal pelvic dilatation labelled obstructive uropathy — most resolve; serial scan, not invasive testing
- A major anomaly counselled without microarray or fetal-medicine referral
A 24-year-old at her routine 20 week scan is told there is "a bright spot on the baby's heart." She is in tears before she reaches your room, convinced the baby has Down syndrome. Your job in the next ten minutes is to put that finding in context — what it is, what it is not, and what (if anything) to do. This is the anatomy scan and the soft markers in their purest, highest-yield form.[1][4]
Overview and definition
The mid-trimester (typically 18 to 20 week) fetal anatomy scan is a screening ultrasound that examines fetal structures systematically, measures biometry, and assesses amniotic fluid, placenta and cervix. Its aims are to detect major structural anomalies, to flag soft markers that modify aneuploidy risk, and to confirm growth and dating. It is a screen, not a diagnostic test — detection is high but never complete, and a normal scan does not guarantee a structurally normal baby.[1][6]
Major structural anomalies affect about 2 to 3% of pregnancies and are a leading cause of perinatal mortality, so the scan is one of the highest-yield interventions in antenatal care.[1]
References9ShowHide
- [1]Salomon LJ, Alfirevic Z, Berghella V, et al. ISUOG Practice Guidelines (updated): performance of the routine mid-trimester fetal ultrasound scan Ultrasound Obstet Gynecol, 2022.PMID 35592929
- [2]International Society of Ultrasound in Obstetrics and Gynecology, Carvalho JS, Allan LD, et al. ISUOG Practice Guidelines (updated): sonographic screening examination of the fetal heart Ultrasound Obstet Gynecol, 2013.PMID 23460196
- [3]Agathokleous M, Chaveeva P, Poon LC, et al. Meta-analysis of second-trimester markers for trisomy 21 Ultrasound Obstet Gynecol, 2013.PMID 23208748
- [4]Lorente AMR, Moreno-Cid M, Rodríguez MJ, et al. Meta-analysis of validity of echogenic intracardiac foci for calculating the risk of Down syndrome in the second trimester of pregnancy Taiwan J Obstet Gynecol, 2017.PMID 28254219
- [5]van Velzen CL, Clur SA, Rijlaarsdam ME, et al. Prenatal diagnosis of congenital heart defects: accuracy and discrepancies in a multicenter cohort Ultrasound Obstet Gynecol, 2016.PMID 26350159
- [6]Salomon LJ, Alfirevic Z, Berghella V, et al. Practice guidelines for performance of the routine mid-trimester fetal ultrasound scan Ultrasound Obstet Gynecol, 2011.PMID 20842655
- [7]Buijtendijk MF, Bet BB, Leeflang MM, et al. Diagnostic accuracy of ultrasound screening for fetal structural abnormalities during the first and second trimester of pregnancy in low-risk and unselected populations Cochrane Database Syst Rev, 2024.PMID 38721874
- [8]Wapner RJ, Martin CL, Levy B, et al. Chromosomal microarray versus karyotyping for prenatal diagnosis N Engl J Med, 2012.PMID 23215555
- [9]Haj Yahya R, Roman A, Grant S, et al. Antenatal screening for fetal structural anomalies - routine or targeted practice? Best Pract Res Clin Obstet Gynaecol, 2024.PMID 38997900