What are the causes of fundal height discrepancy in pregnancy?

Causes of Fundal Height Discrepancy in Pregnancy

Fundal height discrepancy occurs when the measured symphysis-fundal distance deviates from expected gestational age, and the primary causes include fetal growth restriction, multiple gestations, polyhydramnios or oligohydramnios, incorrect dating, maternal obesity, uterine fibroids, abnormal fetal lie, and physiologic lightening in late pregnancy.

Pathological Causes Requiring Immediate Ultrasound Evaluation

Fetal Growth Restriction (FGR)

• Fundal height >3 cm less than gestational age in weeks (McDonald rule) mandates immediate ultrasound with biometry and umbilical artery Doppler studies 1, 2
• FGR is defined as estimated fetal weight (EFW) or abdominal circumference (AC) below the 10th percentile 1
• Placental insufficiency from chronic hypertension, pre-eclampsia, or antiphospholipid syndrome accounts for 50-70% of early-onset FGR cases 1
• Oligohydramnios (amniotic fluid index <5 cm or maximum vertical pocket <2 cm) often accompanies FGR and reflects chronic placental compromise 1

Multiple Gestations

• Twin pregnancies demonstrate substantially greater mean fundal height than singleton pregnancies, though combined birth weight may be less in undiagnosed twins 3
• Fundal height measurements are less reliable in multiple gestations due to variable fetal positioning and should prompt ultrasound rather than reliance on tape measurement 4, 2
• Crown-rump length (CRL) discordance between twins at 11-14 weeks can predict subsequent growth restriction, with 100% demise risk when discrepancy exceeds 60% 4

Polyhydramnios

• Excessive amniotic fluid causes fundal height to exceed expected gestational age 1
• Associated with fetal anomalies, maternal diabetes, and twin-to-twin transfusion syndrome in monochorionic pregnancies 4

Incorrect Gestational Dating

• Pelvic examination in the first trimester correlates more closely with true gestational age than later abdominal fundal height assessments 5
• Ultrasound dating should be used to confirm gestational age when fundal height discrepancy is identified 1, 2

Maternal Factors That Reduce Measurement Reliability

Obesity and Increased Body Mass Index

• Fundal height measurements are unreliable in women with obesity; proceed directly to ultrasound rather than relying on tape measurement 6, 1, 2
• Manual palpation accuracy drops significantly with increased abdominal adiposity, though ultrasound fundal height maintains equivalent predictive power 7

Uterine Fibroids

• Fibroid uterus distorts fundal contour and makes tape measurement unreliable 6, 1, 2
• Ultrasound evaluation is mandatory in these patients for accurate fetal growth assessment 2

Abdominal Distention

• Bowel gas, ascites, or other causes of abdominal distention reduce fundal height measurement accuracy 6

Fetal Position and Presentation Abnormalities

Non-Longitudinal Lie

• Transverse or oblique fetal lie causes fundal height to measure less than expected gestational age 1, 2
• Ultrasound is required to assess fetal position and growth accurately 2

Breech Presentation

• Breech position may alter fundal height measurements compared to cephalic presentation 4

Physiologic Variation in Late Pregnancy

Lightening (Fetal Head Engagement)

• In the last month of pregnancy (after 36 weeks), fundal height may decrease from 36 cm to approximately 32 cm as the fetal head descends into the pelvis 6
• This physiologic drop occurs when the presenting part engages, particularly in primigravidas 2-4 weeks before delivery 6
• This normal descent should not be confused with pathological causes of decreased fundal height 6
• The fundus reaches the xiphoid process at 36 weeks, then drops as engagement occurs 2

Technical and Measurement Factors

Biological Variability in Maternal Anatomy

• Wide variations exist in maternal abdominal length, umbilical position, and uterine fundal level at each gestational age 5, 8
• Anatomic landmarks (umbilicus at 20 weeks, xiphoid at 36 weeks) show significant inter-individual variability 6, 8

Measurement Technique Errors

• Single measurements are less useful than tracking growth trajectory over time on customized charts 2
• Fundal height correlation with gestational age holds best between 18-32 weeks, with reduced accuracy outside this window 7, 9

Clinical Action Algorithm

When fundal height shows discrepancy:

1. Immediate ultrasound if fundal height is >3 cm less than gestational age, <10th percentile on customized charts, or shows <5 mm change over 14 days 1, 2

2. Comprehensive ultrasound assessment must include:

   • Estimated fetal weight and abdominal circumference 1
   • Umbilical artery Doppler velocimetry 1, 2
   • Amniotic fluid volume assessment 1
   • Detailed anatomic survey if early-onset FGR (<32 weeks) is suspected 1

3. For confirmed FGR with normal Doppler: serial ultrasound every 2 weeks, weekly umbilical artery Doppler, weekly non-stress testing after viability, and delivery at 38-39 weeks 1

4. For abnormal umbilical artery Doppler (PI >95th percentile): weekly Doppler evaluation, twice-weekly cardiotocography, and delivery at 37 weeks 1

5. For severe FGR (EFW <3rd percentile): intensified surveillance regardless of Doppler findings and delivery at 37 weeks 1

Critical Pitfalls to Avoid

• Do not delay umbilical artery Doppler assessment once FGR is suspected; Doppler distinguishes pathological restriction from constitutional smallness and determines surveillance intensity 1
• Avoid relying on fundal height alone in obese patients or those with fibroids—proceed directly to ultrasound 1, 2
• Do not use outdated population charts that may not reflect current maternal body habitus 2
• Recognize that up to 20% of early-onset FGR cases have fetal malformations or chromosomal abnormalities requiring detailed anatomic survey and consideration of chromosomal microarray analysis 1
• Remember that approximately 18-22% of fetuses with EFW/AC <10th percentile are constitutionally small with normal outcomes—Doppler assessment is essential to avoid unnecessary intervention 1