Fetal Biometry and Growth Charts
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Direct answer
Fetal biometry is the ultrasound measurement of fetal size, using biparietal diameter, head circumference, abdominal circumference and femur length to estimate fetal weight. Abdominal circumference is the most useful single parameter for growth assessment, because it reflects liver size and glycogen and fat stores. Estimated fetal weight is compared against growth charts to identify small or large fetuses, ideally using population-specific standards.
What you must remember
- BPD: Biparietal diameter, measured at the level of the thalami and cavum septi pellucidi.
- HC: Head circumference, measured as an ellipse around the skull outline, reflecting brain growth.
- AC: Abdominal circumference, measured at the stomach and portal sinus, the most sensitive parameter for growth restriction.
- FL: Femur length, measured without the epiphysis, useful for dating and skeletal size.
- EFW: Estimated fetal weight, calculated from several parameters using a formula such as Hadlock's.
- SGA: Estimated weight or abdominal circumference below the 10th percentile, and severe SGA below the 3rd percentile.
- LGA and macrosomia: Large for gestational age means estimated weight above the 90th percentile; macrosomia is usually a birth weight of 4 kg or more, with thresholds varying by guideline.
What each parameter measures and when it helps
Each parameter answers a different question, and growth is judged across several measurements rather than one reading. The table summarises what each one measures and how it is used.
| Parameter | Measures | Clinical use |
|---|---|---|
| BPD | Width of the skull at the thalami | Gestational age and head growth |
| HC | Outline of the skull | Head growth, reflects brain size |
| AC | Abdomen at stomach and portal sinus | Most sensitive for growth restriction |
| FL | Length of femur without epiphysis | Dating and skeletal size |
| EFW | Weight calculated from the parameters above | Compared with a growth chart for SGA |
Serial measurements, commonly at intervals of three to four weeks, separate constitutional smallness from true growth restriction. Doppler studies are added when growth is abnormal.
Measurements are checked against a reference range rather than read in isolation, and the gestational age used for comparison should be confirmed first. Serial scans add more value than a single estimate. Abnormal results are discussed with the clinical picture, including blood pressure and maternal weight.
Where the exam tries to catch you
Questions test the parameter for growth, the difference between SGA and growth restriction, and the limits of dating and weight estimates.
- Trap: Dating a pregnancy from abdominal circumference. Correct: Abdominal circumference is unreliable for dating, so dating uses crown-rump length in the first trimester and BPD or femur length later.
- Trap: Labelling a small fetus as growth-restricted from one scan. Correct: Growth restriction is diagnosed on serial measurements and Doppler, since a single low weight may be constitutional or caused by dating error.
- Trap: Applying a generic chart without considering population differences. Correct: Population-specific or customised charts are preferred, because birth weights vary by population and maternal characteristics.
- Trap: Treating estimated fetal weight as the exact birth weight. Correct: Estimated fetal weight carries a margin of error that depends on operator skill and maternal body habitus.
Frequently asked questions
Which fetal biometry parameter is best for assessing growth?
Abdominal circumference is considered the most useful single parameter for assessing fetal growth. It reflects liver size and fat and glycogen stores, which fall early in growth restriction. Abdominal circumference is therefore central to the diagnosis of small for gestational age.
What is the difference between SGA and fetal growth restriction?
Small for gestational age means the estimated weight or abdominal circumference is below the 10th percentile, and the baby may simply be constitutionally small. Fetal growth restriction is a pathological failure to reach growth potential. Its diagnosis needs serial scans, Doppler and clinical criteria under current guidance.
How is estimated fetal weight calculated?
Estimated fetal weight is calculated from biparietal diameter, head circumference, abdominal circumference and femur length using a regression formula such as Hadlock's. The result is a single estimate with a margin of error. It is then compared against a growth chart for the gestational age.
Why are population-specific growth charts preferred?
Population-specific growth charts reflect the expected size of babies in a given community, which varies by ethnicity and nutrition. Using them reduces the misclassification of normal Indian fetuses as small. Customised charts also adjust for maternal height, weight and parity.