Daily Fetal Weight Gain
Estimated fetal weight (EFW) increases by approximately 200-220 grams per week in the third trimester, which translates to roughly 28-31 grams per day during this period of rapid growth.
Gestational Age-Specific Growth Rates
The rate of fetal weight gain varies significantly by gestational age and is not constant throughout pregnancy:
Second Trimester Growth
- Growth is slower in mid-pregnancy, with fetuses gaining approximately 85-100 grams per week (12-14 grams per day) between 20-28 weeks gestation 1
- The growth trajectory shows an asymmetric distribution, with wider variation among smaller fetuses in early second trimester 2
Third Trimester Growth (Peak Growth Period)
- Between 28-36 weeks, fetal weight gain accelerates to approximately 200-220 grams per week (28-31 grams per day), representing the period of most rapid growth 1
- At 31 weeks gestation specifically, the mean EFW is approximately 1500 grams, with normal range of 1400-1700 grams 1
- Growth velocity peaks during this period to support the dramatic weight accumulation needed before birth 2
Late Third Trimester (After 36 Weeks)
- Growth rate begins to decelerate after 36 weeks, dropping to approximately 150-180 grams per week (21-26 grams per day) as the fetus approaches term 1, 2
- This physiologic slowing reflects placental limitations and preparation for delivery 2
Important Clinical Considerations
Population Variation
- These growth rates represent population averages and show remarkable physiologic variation even under optimal maternal conditions 2
- The 5th-95th percentile range for EFW at 37 weeks is 2205-3538 grams, demonstrating the wide normal variation in human fetal growth 2
- Maternal factors (height, weight, age, parity) and fetal sex each contribute 1-4.5% to weight variation, with effects graded across percentiles 2
Growth Assessment Intervals
- Serial fetal biometry should be performed at intervals of no less than 2 weeks to reliably detect growth abnormalities, as shorter intervals may not capture meaningful changes given measurement variability 3
- For monochorionic twins at risk for complications, assessment every 2-3 weeks is recommended starting at 16 weeks 3
- For dichorionic twins without complications, evaluation every 3-4 weeks is typically sufficient 3
Clinical Pitfall: Measurement Accuracy
- Ultrasound EFW has inherent measurement error of approximately 10-15%, which must be considered when interpreting growth velocity 4, 5, 6
- The accuracy of weight estimation is affected by maternal body mass index, with higher BMI associated with greater estimation error in term pregnancies 6
- Different EFW formulas may systematically over- or underestimate weight, particularly at extremes of fetal size 5, 7