What are the normal fetal biometry values (biparietal diameter, head circumference, abdominal circumference, femur length, estimated fetal weight) and uterine fundal height at 29 weeks gestation in a normally growing singleton?

Normal Fetal Parameters at 29 Weeks Gestation

At 29 weeks gestation, normal fetal biometry includes a biparietal diameter of approximately 73-77 mm, head circumference of 265-275 mm, abdominal circumference of 240-260 mm, femur length of 54-58 mm, and estimated fetal weight of 1200-1400 grams, with fundal height measuring approximately 27-31 cm (within 3 cm of gestational age in weeks). 1, 2, 3

Fetal Biometric Measurements

Head Parameters

• Biparietal diameter (BPD): The outer-inner measurement at 29 weeks typically ranges from 73-77 mm, representing approximately the 10th to 90th percentile for gestational age 3, 4.
• Head circumference (HC): Normal values at 29 weeks range from 265-275 mm, with HC being a more reliable parameter than BPD when skull shape variations exist 3, 5.

Body Parameters

• Abdominal circumference (AC): At 29 weeks, AC typically measures 240-260 mm and is the single most useful dimension to evaluate fetal growth 3, 5.
• Femur length (FL): Normal femur length at 29 weeks ranges from 54-58 mm, serving as the best parameter for evaluating skeletal development 3, 5.

Estimated Fetal Weight

• EFW at 29 weeks: Normal estimated fetal weight ranges from approximately 1200-1400 grams (10th to 90th percentile), calculated using regression equations combining BPD, HC, AC, and FL 6, 2, 3.
• Weight estimation accuracy: Formulas based on 3-4 fetal biometric indices are significantly more accurate than those using 1-2 indices for birthweights in the 1000-4500 g range 6.

Fundal Height Assessment

Normal Fundal Height Range

• Expected measurement: At 29 weeks, fundal height should measure approximately 27-31 cm (within 3 cm of gestational age in weeks using the McDonald rule) 1.
• Clinical significance: A fundal height measurement ≥3 cm below gestational age (i.e., <26 cm at 29 weeks) requires immediate ultrasound evaluation for suspected fetal growth restriction 1.

Limitations of Fundal Height

• Reliability factors: Fundal height measurements are less reliable in women with obesity, fibroid uterus, multiple gestations, or non-longitudinal fetal lie 1, 7.
• Growth monitoring: A change in fundal height of ≤5 mm over a 14-day interval indicates static or slow growth and warrants ultrasound assessment 1.

Growth Assessment Considerations

Definition of Normal Growth

• Size thresholds: Fetal growth restriction is defined as EFW or AC below the 10th percentile for gestational age, while severe FGR is defined as EFW below the 3rd percentile 6, 1, 2.
• Population-based standards: The American College of Obstetricians and Gynecologists recommends using population-based growth references (such as Hadlock curves) rather than customized standards for accurate diagnosis 1, 2.

Early vs. Late-Onset Growth Issues

• 29 weeks classification: At 29 weeks gestation, any diagnosed growth restriction would be classified as early-onset FGR (diagnosed before 32 weeks), which is typically more severe and associated with placental dysfunction 6.
• Associated risks: Early-onset FGR at this gestational age has higher rates of chromosomal abnormalities (up to 20%) and congenital malformations, requiring detailed anatomic survey 1, 2.

Clinical Surveillance at 29 Weeks

High-Risk Pregnancy Monitoring

• Doppler assessment timing: For high-risk pregnancies with maternal hypertension, prior FGR, or antiphospholipid syndrome, umbilical artery Doppler surveillance should be initiated at 26-28 weeks and continued through delivery 1, 7.
• Surveillance frequency: When FGR is diagnosed at 29 weeks with normal Doppler, weekly umbilical artery Doppler and weekly non-stress testing are recommended 1, 2.

Amniotic Fluid Assessment

• Normal parameters: Amniotic fluid index should be ≥5 cm or maximum vertical pocket ≥2 cm at 29 weeks 6, 1.
• Clinical significance: Oligohydramnios (AFI <5 cm or MVP <2 cm) at this gestational age reflects chronic placental insufficiency and raises suspicion for FGR 1.

Common Pitfalls to Avoid

Measurement Accuracy

• Quality assurance: Implementation of quality-assurance programs for ultrasound measurements is essential to maintain accuracy, with averaging of multiple measurements improving reliability 6, 2.
• Dating prerequisite: Accurate pregnancy dating using first-trimester crown-rump length is essential before interpreting 29-week biometry, as gestational age errors will lead to misclassification of growth status 6, 2, 3.

Clinical Decision-Making

• Multiple parameters: Measurement of multiple fetal parameters (BPD, HC, AC, FL) provides better assessment than relying on a single measurement, as errors in the same direction across all parameters are unlikely 8, 5.
• Doppler integration: Do not delay umbilical artery Doppler assessment if FGR is suspected at 29 weeks, as Doppler is essential for distinguishing pathological restriction from constitutional smallness 1, 7.