Normal Fetal Weight Gain Between 31 and 33 Weeks
Between 31 and 33 weeks of gestation, normal fetal weight gain is approximately 175-190 grams per week, which translates to roughly 350-380 grams total over this 2-week period.
Expected Growth Rate in Late Third Trimester
The fetal growth pattern during this gestational window follows a predictable trajectory:
Average weekly weight gain in the third trimester is approximately 176.5 g/week (SD 29.1), based on calculations from term birth weights extrapolated backward 1.
Daily weight gain from 28 weeks onward is constant at approximately 27 g/day, which equals roughly 189 g/week 2. This linear growth pattern continues until several weeks after term.
Over the 2-week interval from 31 to 33 weeks, this translates to an expected total weight gain of approximately 350-380 grams (using 175-190 g/week as the reference range).
Clinical Context for Growth Assessment
When evaluating fetal growth during this period, several key principles apply:
Growth velocity assessment requires attention to downward trends in abdominal circumference (AC) or estimated fetal weight (EFW) centiles, as slowing growth velocity indicates increased risk of morbidity and stillbirth 3.
At gestational age ≥32 weeks, late fetal growth restriction (FGR) is defined by specific criteria including AC/EFW <3rd centile, or at least two of three findings: AC/EFW <10th centile, AC/EFW crossing centiles >two quartiles, or cerebroplacental ratio <5th centile 3.
Suboptimal growth is defined as AC/EFW crossing centiles greater than two quartiles on non-customized growth centiles at ≥32 weeks (equivalent to 50 centiles), or static growth with no forward velocity in EFW/AC measured at least 14 days apart 3.
Factors Affecting Growth Rate
Several maternal and fetal factors influence weight gain during this period:
Fetal sex affects growth velocity, with male fetuses gaining approximately 9 g/week more than females 1.
Maternal parity influences growth, with multiparous women having fetuses with approximately 7 g/week higher weight gain 1.
Maternal smoking significantly reduces fetal growth velocity (p<0.0001) 1.
Important Clinical Caveats
Ultrasound measurement error is substantial, with test-retest variation as much as 50% of the standard deviation of gestational-age-specific fetal weight distribution 2. This means growth must be followed for many weeks to document statistically significant deviations.
Serial measurements should be spaced at least 14 days apart to allow meaningful assessment of growth velocity 3.
All fetal biometry measurements since the anomaly scan should be reviewed when growth concerns arise, to identify potentially erroneous single measurements and assess for downward trends 3.