What is the anatomical relationship between fundal height and gestational age?

Anatomical Relationship Between Fundal Height and Gestational Age

The fundal height in centimeters measured from the pubic symphysis to the top of the uterine fundus approximately equals gestational age in weeks, with this correlation holding most reliably between 18-32 weeks of gestation. 1

Standard Measurement Correlation

• Fundal height should be measured using a tape measure from the pubic symphysis to the uterine fundus, with the measurement in centimeters directly corresponding to gestational age in weeks during the 18-32 week window. 1
• The rate of fundal growth is approximately 1 cm per week between 20-32 weeks gestation, after which the growth rate slightly decreases. 2
• Research confirms this relationship holds in both manual palpation and ultrasound measurements, with strong correlation (R² = 0.92) between the two methods. 3

Key Anatomical Landmarks by Gestational Age

• At 12 weeks, the uterus becomes palpable above the pubic symphysis. 4
• At 20 weeks, the fundus reaches the level of the umbilicus. 4, 5
• At 36 weeks, the fundus reaches the xiphoid process (xiphisternum). 1, 4
• In the last month of pregnancy (after 36 weeks), fundal height may decrease from approximately 36 cm down to 32 cm as the fetal head engages into the pelvis during "lightening," particularly in first-time mothers. 4

Clinical Application and Charting

• Serial fundal height measurements should be plotted on customized growth charts that account for maternal characteristics, as this approach has been associated with improved detection of small-for-gestational-age babies and reduction in stillbirth. 1
• All six major international guidelines (United Kingdom, New Zealand, Canada, Ireland, United States, France) recommend serial fundal height measurement as a screening tool for fetal growth abnormalities. 6
• Single measurements are less clinically useful than tracking growth trajectory over time on a chart. 1

Defining Abnormal Growth

• Suboptimal fetal growth is defined by the McDonald rule as fundal height measurement >3 cm less than gestational age in weeks. 6, 1
• When fundal height lags >2-3 cm behind expected gestational age, ultrasound evaluation for fetal growth restriction should be performed. 1
• Serial measurements showing inadequate growth velocity warrant ultrasound evaluation even if absolute measurements remain within normal range. 1

Important Clinical Limitations

• Fundal height measurements are unreliable in women with obesity and/or fibroids, and ultrasound scans should be used instead in these populations. 6, 1
• Body habitus, abdominal distention, and increased body mass index significantly reduce the reliability of fundal height as a predictor of gestational age. 4
• Multiple gestations and non-longitudinal fetal lie also make fundal height measurements less reliable. 4
• Recent meta-analysis data from low-to-middle-income countries shows that using the 1 cm = 1 week assumption, fundal height underestimated gestational age by a mean of 14 days with poor accuracy (95% limits of agreement ±42.8 days), indicating fundal height should not be used for gestational age dating when ultrasound is available. 7

Common Pitfalls to Avoid

• Do not rely on outdated population charts from the 1980s that may not reflect current maternal weights and body habitus. 6, 1
• Do not use fundal height alone in obese patients where sensitivity drops significantly—proceed directly to ultrasound in these cases. 1
• Do not confuse the normal physiological drop in fundal height during lightening (fetal head engagement) with pathological causes of decreased fundal height. 4
• The cervix does not "drop" during lightening—only the fundus demonstrates measurable descent; the cervix undergoes effacement and dilation during labor. 4