Isolated low-normal amniotic fluid volume in the early third trimester: Association with adverse perinatal outcomes

Aims: To test if an isolated finding of low-normal amniotic fluid index (AFI) in the early third trimester in low-risk patients is associated with adverse perinatal outcomes. Methods: Retrospective cohort study with uncomplicated singleton pregnancies that had ultrasound studies between 28.0 and 31.9 weeks ' gestation. Two cohorts with AFI 8.0 - 11.9 cm (low-normal, LN) and AFI 12.0 - 19.9 cm (mid-normal, MN) were compared. Results: Patients with LN-AFI (n=99) were more likely to have early (=34 weeks) and late (=37 weeks) preterm birth (PTB) (relative risk 4.2 and 2.4, respectively) and a small for gestational age (SGA) infant (relative risk 1.8) than MN-AFI (n=834), corresponding to a higher NICU admission rate (relative risk 2.5). The risk of " spontaneous " PTB (preterm labor and rupture of membranes) did not differ between the cohorts, whereas the risk of " indicated " PTB (maternal or fetal indications) was significantly increased in LN-AFI. The incidence of abnormal antepartum testing, stillbirth, preeclampsia, placental abruption, fetal intolerance to labor, emergency cesarean delivery, umbilical artery pH=7.0, Apgar scores=7 at 5 min, and neonatal death was not increased in patients with LN-AFI. Conclusion: Low-normal AFI in the early third trimester increases the risk for subsequent delivery of an SGA infant and indicated PTB.