The unacceptably high stillbirth rate in Nigeria, which results mainly from antepartum fetal death demands evidence-based antenatal interventions. While evidence suggests that fetal surveillance using umbilical artery (UA) Doppler reduces perinatal mortality; normative references of fetal UA Doppler indices are not readily available in Nigeria. This study aims to construct indigenous, gestational age-specific normative references for fetal UA Pulsatility Index (PI), Resistance Index (RI) and Systolic/Diastolic ratio (S:D) of singleton pregnancies in Ile-Ife, Nigeria, and correlate this with pregnancy outcomes.
In a longitudinal study involving 415 women with singleton fetuses between the gestational ages of 26-40weeks at the Obafemi Awolowo University Teaching Hospital, Nigeria, fetal umbilical artery PI, RI and S:D were measured four-weekly till delivery. The means and normative centiles (95% range between 2.5th -97.5th) of the indices were determined. Regression equations were generated for the three indices, and Pearson’s correlation was used to establish the relationship with maternal age, parity, gestational age (GA) and birth weight.
The three indices demonstrated significant inverse correlation with GA, such that the RI reduced by 0.013, while the PI and S:D ratio reduced by 0.027 and 0.71 respectively for each additional week of pregnancy. There was a significant negative correlation between the RI (r =-0.207; p = 0.001), PI (r = -0.17; p= 0.01), S:D ratio (r = -0.25; p<0.001) and birth weight, but not with maternal age and parity. The regression equations were RI=1.004-0.013(x), PI=1.78-0.027(x) and S:D=4.77-0.71(x) (where x= GA in weeks).
The fetal umbilical artery Doppler indices decreased progressively with advancing GA. There was a significant reverse-association with birthweight, but not with maternal age or parity. This study has generated, gestational age-specific umbilical artery Doppler normative references ideal for the local population’s antepartum fetal assessment.