Timing of Biometry, Congenital Anomaly Scan, and Biophysical Profile Based on ACOG Guidelines
The congenital anomaly (anatomy) scan should be performed at 18-20 weeks gestation, fetal biometry is performed at the anatomy scan and repeated as clinically indicated (typically every 2-4 weeks if growth concerns arise), and the biophysical profile (BPP) is reserved for second and third trimester fetal surveillance with no role in the first trimester. 1, 2
Congenital Anomaly (Anatomy) Scan
The standard anatomic ultrasound must be performed at 18-20 weeks gestation as the primary screening window for detecting fetal structural anomalies, as recommended by the American College of Radiology and endorsed by ACOG 2, 3.
This timing represents the optimal balance between fetal size (allowing adequate visualization of structures) and the ability to detect approximately 75% of major congenital anomalies 1.
For obese patients (BMI ≥35 kg/m²), delay the anatomy scan to 20-22 weeks due to suboptimal visualization through increased abdominal wall thickness, with repeat follow-up in 2-4 weeks if the initial scan is incomplete 2, 3.
The anatomy scan should systematically evaluate fetal brain, spine, face, heart, abdomen, kidneys, bladder, extremities, and placental position 2.
Fetal Biometry Timing
Initial Biometry Assessment
Fetal biometry (biparietal diameter, head circumference, abdominal circumference, and femur length) is first performed at the 18-20 week anatomy scan to establish baseline measurements and estimate fetal weight 1, 2.
Crown-rump length between 7-12 weeks provides the most accurate first-trimester dating parameter, but this is for dating purposes rather than growth assessment 4, 5.
Serial Biometry for Growth Monitoring
In preeclampsia, perform fetal biometry at first diagnosis, then repeat every 2 weeks from 24-26 weeks until delivery if fetal growth restriction is present 1, 2.
For multiple gestations, dichorionic twins require serial growth scans every 3-4 weeks starting from the anatomy scan, while monochorionic twins need fetal biometry every 2-3 weeks beginning at 16 weeks due to higher complication rates 2.
The minimum interval between growth scans should be 2 weeks, as more frequent measurements do not improve accuracy and may lead to false-positive diagnoses of growth abnormalities 1.
Special Populations Requiring Serial Biometry
Maternal obesity (BMI ≥35 kg/m²) warrants a growth scan at 28-32 weeks to aid detection of late-onset fetal growth restriction when clinical assessment is limited 2.
When fetal growth restriction is diagnosed (estimated fetal weight <10th percentile with abnormal umbilical artery Doppler), assessment of fetal growth is recommended at 2-week intervals 1.
Biophysical Profile (BPP) Timing
The BPP is performed exclusively in the second and third trimesters; there is no role for BPP in the first trimester 1.
BPP is not a routine screening test but rather a targeted assessment used for fetal surveillance when specific concerns arise, such as decreased fetal movement, suspected fetal compromise, or high-risk conditions 1.
The BPP evaluates five parameters: fetal breathing movements, fetal body movements, fetal tone, amniotic fluid volume, and non-stress test, each scored 0 or 2 points for a maximum of 10 points 1.
First Trimester Ultrasound (Not Anatomy Scan)
A first-trimester ultrasound at 11-14 weeks can assess nuchal translucency for aneuploidy screening, detecting approximately 80-90% of affected pregnancies when combined with maternal serum markers 1.
Some specialized centers perform a detailed first-trimester obstetric ultrasound between 12 weeks 0 days and 13 weeks 6 days, which can detect approximately 27-50% of major fetal anomalies, though this does not replace the standard second-trimester anatomy scan 1.
Nuchal translucency ≥3 mm warrants immediate genetic counseling and consideration of diagnostic testing, as approximately one-third of these fetuses will have chromosomal abnormalities 1.
Multiple Gestation Surveillance Schedule
Determine chorionicity and amnionicidity at the earliest ultrasound (ideally first trimester), as monochorionic twins carry a 10% mortality risk necessitating intensive surveillance 2.
Monochorionic diamniotic twins require surveillance beginning at 16 weeks with weekly-to-biweekly monitoring for twin-twin transfusion syndrome, including assessment of amniotic fluid volumes and bladder visualization 2.
Dichorionic twins follow a less intensive schedule: first-trimester dating scan, nuchal translucency assessment at 11-14 weeks, anatomy scan at 18-22 weeks, then serial growth scans every 3-4 weeks 2.
Common Pitfalls to Avoid
Do not use second or third-trimester ultrasound measurements to recalculate gestational age once accurate dating has been established by first-trimester crown-rump length or early second-trimester measurements 5.
Avoid performing fetal biometry more frequently than every 2 weeks, as shorter intervals do not improve diagnostic accuracy and increase false-positive rates for growth abnormalities 1.
Do not rely on first-trimester ultrasound (even detailed scans at 12-14 weeks) as a substitute for the standard 18-20 week anatomy scan, as sensitivity for anomaly detection is only 50% in the first trimester compared to 75% at 18-22 weeks 1.
Never order a BPP in the first trimester—it has no clinical utility or established role before the second trimester 1.