A 32-year-old primigravida with an uncertain last menstrual period is evaluated at 16 weeks by clinical examination and ultrasound. On abdominal palpation, the fundus is at the level of the symphysis pubis. Ultrasound shows a biparietal diameter (BPD) of 32 mm. Which finding best distinguishes second-trimester ultrasound dating from clinical dating by uterine fundal height in this patient?
A. Ultrasound BPD is unaffected by maternal body habitus and parity; fundal height varies with uterine size, parity, and abdominal wall laxity
B. Ultrasound BPD is accurate to ±2 weeks; fundal height is accurate to ±1 week
C. Ultrasound BPD requires fetal cardiac activity; fundal height can be assessed in any pregnancy
D. Ultrasound BPD is most accurate between 15–20 weeks; fundal height is most accurate in the third trimester
Explanation
Comparison: Second-Trimester Ultrasound Dating vs Clinical Fundal Height Dating
Key Discriminating Feature
Key Point
Ultrasound BPD is an objective biometric measurement independent of maternal factors, whereas fundal height is a clinical estimate heavily influenced by maternal body habitus, parity, and abdominal wall characteristics.
Sources of Error in Fundal Height Assessment
High-YieldNEET PG
Fundal height dating is confounded by:
Maternal obesity — abdominal wall laxity masks true fundal position
Multiparity — uterus is more lax; fundus appears lower than expected for gestation
Maternal height and build — taller women have longer abdomens
Placental position — anterior placenta may reduce apparent fundal height
Polyhydramnios or oligohydramnios — alters uterine distension
Comparison Table
Table
Feature
Ultrasound BPD
Fundal Height
Accuracy
±2 weeks (15–20 weeks)
±3–4 weeks
Confounders
None (objective measurement)
Parity, BMI, maternal height, placental position
Reproducibility
High (operator-dependent but standardized)
Low (subjective, inter-observer variation)
Affected by fetal factors
Genetic variation in head size
No
Affected by maternal factors
No
Yes (body habitus, parity)
Clinical Pearl
In a multiparous woman with lax abdominal wall, fundal height may be 2–3 cm below expected for gestation, leading to underestimation of gestational age. Ultrasound is unaffected by this.
Why Ultrasound Wins in Uncertain Dating
Mnemonic
BIOMETRIC — Body habitus doesn't affect; Imaging objective; Operator-dependent but standardized; Measurement direct; Exact dating; Trophoblast size varies; Reliable in second trimester; Independent of parity; Confounders absent.
1.
Ultrasound measures fetal bone directly
2.
Maternal factors do not alter fetal BPD
3.
Fundal height is subjective and varies widely with parity and maternal body composition
Williams Obstetrics 25e Ch 4; Cunningham's Obstetrics 26e Ch 8
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