Best Time for Fetal Growth Scan
For low-risk pregnancies, a routine growth scan at 36 weeks' gestation is superior to earlier scanning at 32 weeks for detecting fetal growth restriction and small-for-gestational-age infants. 1, 2
Low-Risk Pregnancies
Routine third-trimester ultrasound is not recommended for low-risk women, as 83% of international guidelines agree there is no role for universal screening in this population 1. However, when clinical suspicion arises or a growth scan is indicated:
- 36 weeks' gestation is the optimal timing for detecting fetal growth restriction (FGR), with sensitivity of 38.8% compared to only 22.5% at 32 weeks 1, 2
- For severe FGR (<3rd centile), 36-week scanning achieves 61.4% sensitivity versus 32.5% at 32 weeks 2
- The diagnostic odds ratio for detecting small-for-gestational-age (SGA) infants is significantly higher at 36 weeks (13.3) compared to 32 weeks (8.8) 3
- A single scan at 36 weeks is more cost-effective than two scans at 32 and 38 weeks, with no difference in detection rates 4
High-Risk Pregnancies and Preeclampsia
For pregnancies complicated by preeclampsia or known risk factors for FGR, the approach differs substantially:
- Perform fetal biometry, amniotic fluid assessment, and Doppler studies immediately at diagnosis of preeclampsia, regardless of gestational age 1
- Serial surveillance from 26 weeks' gestation is recommended when preeclampsia is confirmed and pregnancy continuation is planned 1
- Repeat growth scans every 2 weeks once FGR is diagnosed 1
- Increase to weekly surveillance if umbilical artery Doppler shows increased resistance (pulsatility index >95th centile) 1
Clinical Triggers for Growth Scanning
Ultrasound should be considered when:
- Fundal height measurement is >3 cm less than gestational age in weeks 1
- Obesity or fibroid uterus makes fundal height measurements unreliable 1
- Any clinical suspicion of abnormal fetal growth 1
Important Caveats
The sensitivity of ultrasound for detecting FGR remains imperfect even at optimal timing, with 36-week scans detecting only approximately 50% of SGA infants in low-risk populations 1, 2. This limitation must be communicated to patients, as normal ultrasound findings do not guarantee appropriate fetal growth.
Earlier scanning at 32 weeks may miss late-onset FGR, as many growth-restricted fetuses only become identifiable closer to term 1. The interval between scan and delivery significantly affects accuracy, with shorter intervals improving detection 3, 4.
Multiple growth scans do not improve outcomes in low-risk pregnancies and substantially increase costs without enhancing diagnostic effectiveness 4.