Normal AFI at 27 Weeks Gestation
At 27 weeks gestation, the normal Amniotic Fluid Index (AFI) range is 5-24 cm, with values below 5 cm indicating oligohydramnios and above 24 cm indicating polyhydramnios. 1
Gestational Age-Specific Considerations
While the general normal range of 5-24 cm applies across gestation, important nuances exist at 27 weeks:
Before 37 weeks gestation, an AFI of 8 cm or greater is commonly used to define normal amniotic fluid, which is more stringent than the lower threshold of 5 cm used at term 1
Research data from the NICHD Fetal Growth Studies shows that AFI values peak around 26 weeks gestation, making 27 weeks a time of relatively high amniotic fluid volume 2
At 27 weeks specifically, the 3rd percentile AFI is approximately 10.1 cm and the 97th percentile is in the range of 24-25 cm, suggesting that single cutoffs may not capture the full physiologic variation 2
Clinical Interpretation at 27 Weeks
The volume of fluid per centimeter of AFI changes throughout pregnancy, which affects interpretation:
At 20 weeks, each cm of AFI represents approximately 25 mL of fluid, but this increases to about 40 mL per cm by 30 weeks—a 61% increment 3
This means an AFI of 18 cm at 27 weeks represents substantially more actual fluid volume than the same AFI measurement at earlier gestations 3
Diagnostic Thresholds
When evaluating AFI at 27 weeks:
AFI < 5 cm defines oligohydramnios and is independently associated with increased stillbirth risk (odds ratio 2.6) 1, 4
AFI > 24 cm or >95th percentile defines polyhydramnios and is associated with increased perinatal mortality (odds ratio 5.8) 1
Consider using Maximum Vertical Pocket (MVP) ≥ 2 cm as an alternative measurement, as it may result in fewer false diagnoses of oligohydramnios and unnecessary interventions compared to AFI 1, 4
Common Pitfalls to Avoid
Do not apply term pregnancy thresholds (AFI ≥ 5 cm) to preterm gestations like 27 weeks—the threshold of 8 cm is more appropriate before 37 weeks 1
Avoid relying solely on AFI for clinical decisions without considering other fetal surveillance parameters, as isolated AFI abnormalities may lead to overdiagnosis 4
Be aware that racial/ethnic differences exist in AFI measurements, though these differences are most prominent after 35 weeks and may not be clinically significant at 27 weeks 2