RESUMO
OBJECTIVE: To evaluate the relationship between umbilical artery pulsatility index and the umbilical cord pH in small-for-gestational-age fetuses. METHODS: Data were collected from all pregnancies seen in our Fetal Surveillance Unit that underwent antenatal monitoring for being small-for-gestational-age beyond 24 weeks of pregnancy. A linear regression was used to analyse the effect of the umbilical artery pulsatility index (UAPI) and umbilical artery pH at birth, assessing interaction and controlling for clinically meaningful antenatal, Doppler, biophysical and perinatal variables. RESULTS: A total of 117 pregnancies were included. According to the estimated model (which included all clinically significant confounders and statistically significant interactions), in cases with reactive cardiotography (CTG) a 1-unit increase in the UAPI resulted in a mean decrease of 0.021 (95% 0.05 to -0.005) units of umbilical artery pH at delivery. In the non-reactive CTG group a 1-unit increase in the UAPI resulted in a mean decrease of 0.02 (95% CI 0.059 to -0.019) units of umbilical artery pH at delivery. Finally, in fetuses with a pathological CTG a 1-unit increase in the UAPI resulted in a mean decrease of 0.149 (95% CI 0.107 to 0.19) units of umbilical artery pH at delivery. CONCLUSIONS: The crude effect between umbilical artery velocimetry on umbilical artery pH at birth has to be interpreted with caution, since an important part of this effect is related to other factors that confound and modify the relationship. After controlling for confounders, it was only in fetuses with pathological CTG that the effect remained statistically significant.
