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.