Mid-trimester uterine artery resistance measured with Doppler sonography is predictive for iatrogenic preterm birth. In view of the emerging association between hypertensive disease in pregnancy and spontaneous preterm birth, we hypothesized that uterine artery resistance could also predict spontaneous preterm birth.
We performed a cohort study studying women with singleton pregnancies. Uterine artery resistance was routinely measured at the 18-22 weeks anomaly scan. Pregnancies complicated by congenital anomalies or intra-uterine fetal death were excluded. We analyzed if the waveform of the uterine artery (no notch, unilateral notch or bilateral notch) was predictive for spontaneous and iatrogenic preterm birth, defined as delivery before 37 weeks. Furthermore, we assessed whether the uterine artery Pulsatility Index was associated with the risk of preterm birth.
Between January 2009 and December 2016 we collected uterine Doppler indices and relevant outcome data in 4,521 women. Mean gestational age at measurement was 19+6 weeks. There were 137 (3.0%) women with a bilateral and 213 (4.7%) with a unilateral notch. Mean gestational age at birth was 38+6 weeks. Spontaneous and iatrogenic preterm birth rates were 5.7% and 4.9% respectively. Mean uterine artery resistance was 1.12 in the spontaneous preterm birth group compared to 1.04 in the term group (p 0.004) The risk of preterm birth was increased with high uterine artery resistance (OR 2.9 per unit; 95% CI; 2.4 to 3.9). Prevalence of spontaneous preterm birth increased from 5.5% in women without notch in the uterine arteries to 8.0% in women with a unilateral notch and 8.0% in women with a bilateral notch. For iatrogenic preterm birth, these rates were 3.9%, 13.6% and 23.4% respectively. Likelihood ratios for the prediction of spontaneous preterm birth were 1.6 (95% CI; 1.0 to 2.6) and 1.9 (95% CI; 1.0 to 3.5) for unilateral and bilateral notches, respectively, and for iatrogenic preterm birth 3.6 (95% CI; 2.5 to 5.2) and 6.8 (95% CI; 4.7 to 9.9) for unilateral and bilateral notches. Of all women with bilateral notching 31.4% delivered preterm.
Mid-Trimester uterine artery resistance measured at 18-22 weeks of gestation is a weak predictor of spontaneous preterm birth.