The effect of low-dose aspirin on fetal urine output and amniotic fluid volume

Abstract

Objective: Patients given prostaglandin synthetase inhibitors in doses sufficient to inhibit labor are at risk for developing oligohydramnios (possibly related to a reduction in fetal urine output). We sought to ascertain whether the fetuses of women who received 60 mg of aspirin daily had a lower urine output than those whose mothers were given a placebo.

Study design: Nulliparous women with singleton gestations in a double-blind preeclampsia prevention trial were randomly selected at 24 weeks' gestation to receive either 60 mg of aspirin daily or a placebo. Urine output was assessed in 59 fetuses (aspirin 32, placebo 27) by serial ultrasonographic measurement of their bladder volume (volume = 4/3 pi r3). Biochemical evidence of aspirin compliance was defined as an 80% reduction in maternal serum thromboxane B2 levels when comparing values obtained at randomization with those at 34 to 36 weeks' gestation.

Results: Visual assessment of amniotic fluid volume was similar in both groups. Four-quadrant amniotic fluid indexes also were similar (13.5 cm in aspirin group vs 12.2 cm in placebo group, p = 0.15). Mean fetal urine outputs were similar in the aspirin (57.7 ml/hr) and placebo (55.1 ml/hr) groups (p = 0.71). Moreover, the 23 women with a fourfold thromboxane B2 reduction had a higher mean fetal urine output (63.5 vs 51.8 ml/hr, p = 0.08) than did the remaining 35 patients. This study has a 96% chance (1-beta) of detecting a 50% (30 ml) reduction in fetal urine output.

Conclusions: Daily maternal ingestion of 60 mg of aspirin did not decrease fetal urine output or amniotic fluid volume.