Maternal hyperoxygenation: a fetal blood flow velocity prognosis test in small-for-gestational-age fetuses?

Maternal hyperoxygenation tests were performed in 20 small-for-gestational age fetuses using 8 l/min of 70% humidified oxygen given for 20 min through a face mask. Assessments were made of the fetal blood flow velocity, and the placental resistance index, cerebral resistance index and cerebroplacental ratio were calculated in 78 normal pregnancies and in the 20 small-for-gestational-age fetuses during the hyperoxygenation test. The test was positive when the velocity indices increased by more than 10%. The reference ranges for placental and cerebral resistance indices were derived from the data measured in the 78 normal pregnancies. Of the 20 small-for-gestational-age fetuses, three had a normal placental resistance index with a negative oxygen test response and a good fetal outcome. Seven had a mild increase in placental resistance index (> +2SD) and a brain-sparing effect was observed (cerebral resistance index < -2SD or cerebroplacental ratio < l). In these seven fetuses, the fetal outcome was not poor, whether the oxygen test response was negative or positive. Ten of the 20 small-for-gestational-age fetuses had no umbilical diastolic flow (placental resistance index = 1) and a brain-sparing effect. Their outcome was strongly dependent on the oxygen test response: a positive response indicated a good prognosis, a negative one a poor fetal outcome.