Expected Fetal Weight Gain in the Third Trimester
During the third trimester, a fetus typically gains approximately 0.5 kg (500 grams) per week, with total weight gain of about 5-7 kg during this period. 1
Normal Fetal Growth Pattern in Third Trimester
Fetal growth in the third trimester follows a relatively linear pattern, with the most significant weight gain occurring during this period compared to earlier trimesters. The specific weekly weight gain patterns include:
- Average weekly weight gain: Approximately 176.5 g/week (SD 29.1) during the third trimester 2
- Total third trimester gain: 5-7 kg total, representing the majority of the fetal weight gain throughout pregnancy
- Growth velocity: Quasi-linear growth pattern in the third trimester 2
Factors Affecting Third Trimester Fetal Growth
Several maternal and fetal factors influence the rate of fetal weight gain:
Maternal BMI: Pre-pregnancy BMI significantly affects fetal growth patterns 1
- Underweight women (BMI <19.8 kg/m²): Fetuses may have higher growth velocity
- Overweight women (BMI >26 kg/m²): Fetuses may have lower growth velocity
Maternal nutrition: Adequate protein and energy intake directly impacts fetal growth 1
- Most protein deposition occurs in the third trimester
- Approximately 40% of maternal protein gain is represented by the fetus, placenta, and amniotic fluid
Maternal weight gain: Third trimester maternal weight gain correlates with fetal growth 3
- Excessive maternal weight gain increases risk of large-for-gestational-age (LGA) infants
- Insufficient maternal weight gain increases risk of small-for-gestational-age (SGA) infants
Clinical Significance of Third Trimester Growth
The third trimester represents a critical period for fetal weight gain:
Growth restriction risk: Low maternal weight gain in the third trimester is associated with a 1.7-2.5 times increased risk of intrauterine growth restriction 4
Monitoring implications: Assessment of fetal growth should be performed at intervals of 2-4 weeks during the third trimester to ensure appropriate growth velocity 1
Nutritional requirements: Energy needs increase by approximately 300 kcal/day during the third trimester to support fetal growth 1
Common Pitfalls in Assessing Third Trimester Growth
When evaluating fetal growth during the third trimester, be aware of these important considerations:
Growth curve selection: Use population-specific growth curves that best represent the demographic being assessed 1
Measurement frequency: Fetal biometry assessments should be spaced at least 2 weeks apart, with 3-4 weeks being optimal for reliable growth assessment 1
Constitutional vs. pathological SGA: Not all small fetuses are growth restricted; some are constitutionally small but growing appropriately along their growth curve 1
Gender differences: Male fetuses typically have higher average weekly weight gain (approximately 9g/week more) than female fetuses 2
The third trimester represents the most critical period for fetal weight gain, and monitoring growth during this time is essential for identifying potential complications and ensuring optimal outcomes.