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

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.

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.

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.

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