Small-for-Gestational-Age Fetus and a Growth Restricted Fetus, Investigation and Care
Definitions
- Small-for-gestational-age (SGA): estimated fetal weight (EFW) or abdominal circumference (AC) below the 10th centile for gestational age. Severe SGA: below the 3rd centile.
- Fetal growth restriction (FGR): a fetus that has failed to reach its own genetically-determined growth potential — not simply small, but demonstrating a pathological restriction in growth velocity. Most FGR fetuses are SGA, but not all SGA fetuses are FGR — a proportion are constitutionally small and healthy.
- The guideline's central emphasis: distinguish constitutionally small fetuses (low risk) from those with true growth restriction (raised risk of stillbirth, neonatal morbidity, and long-term adverse outcomes) — surveillance intensity should follow the distinction, not centile alone.
Risk Factors and Screening
- Major risk factors (warrant serial growth scans from 26-28 weeks): previous SGA/stillbirth, maternal smoking, cocaine use, daily vigorous exercise, previous pre-eclampsia, maternal age ≥40, heavy antepartum bleeding, low PAPP-A, echogenic bowel, unexplained APH.
- Minor risk factors (individually less predictive; three or more warrant uterine artery Doppler at 20-24 weeks): maternal age ≥35, IVF singleton pregnancy, nulliparity, BMI <20 or 25-34.9, smoking 1-10 cigarettes/day, low fruit intake pre-pregnancy.
- Symphysis-fundal height (SFH) measurement and plotting on a customised growth chart from 24 weeks is the recommended screening tool for the low-risk population; serial ultrasound is used where risk factors are present or SFH is discrepant.
Surveillance
- Ultrasound biometry: EFW/AC plotted on a customised centile chart; serial measurements at least 2 weeks apart to assess growth velocity (a single scan cannot distinguish SGA from FGR).
- Umbilical artery Doppler is the primary surveillance tool once SGA is suspected or confirmed — raised resistance, absent, or reversed end-diastolic flow (AREDF) indicates placental insufficiency and escalates surveillance/delivery timing.
- Middle cerebral artery (MCA) Doppler and ductus venosus Doppler are used in more severe or earlier-onset cases to assess redistribution ("brain-sparing") and cardiovascular decompensation.
- Amniotic fluid volume — oligohydramnios supports a diagnosis of placental insufficiency in an SGA fetus.
Management and Timing of Birth
- Normal umbilical artery Doppler: continue surveillance, deliver by 37 weeks if SGA persists without other concerning features.
- Abnormal umbilical artery Doppler but end-diastolic flow present: increase surveillance frequency; deliver by 37 weeks.
- Absent end-diastolic flow (AEDF): deliver by 34 weeks (after antenatal corticosteroids — see RCOG GTG 74).
- Reversed end-diastolic flow (REDF): deliver by 32 weeks.
- Antenatal corticosteroids should be given wherever delivery before 34+0 weeks is anticipated, per GTG 74.
- Continuous electronic fetal monitoring is recommended during labour for a confirmed SGA/FGR fetus, given the higher risk of intrapartum hypoxia.
High-Yield Exam Points
- SGA ≠ FGR: the distinction (constitutional smallness vs pathological growth restriction) is the single most commonly tested concept from this guideline.
- Umbilical artery Doppler is the primary surveillance investigation — know the escalation pathway from normal → absent → reversed end-diastolic flow and the corresponding delivery timing thresholds (34 weeks for AEDF, 32 weeks for REDF).
- Customised growth charts (accounting for maternal height, weight, parity, ethnicity) are recommended over population-average charts to reduce false positive/negative SGA classification.
Source: RCOG Green-top Guideline No. 31 (3rd edition) (Reviewed 14 May 2024; next review scheduled 2027)
Read the original on rcog.org.uk
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