Previous studies have shown that noninvasive assessment of fetal pulmonary circulation is feasible and reproducible using Doppler ultrasound with analysis of pulmonary artery (PA) spectral waveforms.1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 Deviations in specific PA Doppler parameters have been reported in fetuses that experienced certain pulmonary diseases. Fetuses with lung hypoplasia– (LH-) related conditions tend to exhibit increased pulsatility index (PI),12, 13, 14 decreased peak systolic velocity (PSV),13, 14, 15 shortened acceleration time (AT),13 and lower AT–to–ejection time (ET) ratio (AT/ET).16,17 Contradictory AT and AT/ET findings have been made among fetuses with neonatal respiratory distress syndrome.18, 19, 20, 21, 22 In addition, there is a growing interest in studying fetal pulmonary vascular reactivity during brief maternal hyperoxygenation (MH), which decreases the fetal pulmonary vascular resistance (PVR) and can mimic immediate postnatal physiologic transition, to determine the prognosis of fetuses with LH and congenital heart diseases (CHDs). Fetuses with LH,23 hypoplastic left heart syndrome (HPLS),24,25 or dextrotransposition of the great artery24 who exhibited insignificant reduction in PA PI during MH (no significant decreased PVR) usually die of LH or need urgent neonatal atrial septostomy from restricted or intact atrial septum.

In utero, pulmonary vasculature and cardiac outputs typically change as the fetus grows and gestation advances. These physiological changes affect the fetal PA spectral waveform via alterations in PVR, cardiac output, and total pulmonary blood flow. To assess PA spectral Doppler parameters in fetuses with pulmonary diseases or CHDs at different gestational ages (GAs), GA-specific reference parameter value ranges are needed. Although several reference ranges for human fetal PA Doppler parameters during the second and third trimesters of pregnancy have been published,1, 2, 3, 4, 5, 6, 7, 8,10,11 the reported reference ranges exhibited several inconsistencies. For example, Laudy et al.2 and Yamamoto et al.8 showed that PI did not change as pregnancy advance, while Chaoui et al.3 and Liu et al.11 reported progressive decreases in PI. Similarly, while Rasanen et al.1 and Chaoui et al.3 reported lengthening of AT and increasing AT/ET from midpregnancy until term, Fuke et al.16 indicated that GA did not affect AT/ET. An explanation for these disagreements may be the small sample sizes in these studies, which range from 86 to 273 fetuses in the second half of pregnancy—meaning that there were approximately only 4 to 14 fetuses representing each gestational week. In addition, to the best of our knowledge, thus far only a few studies have been conducted with the primary aim of establishing a standard score or percentile ranking solution for fetal PA Doppler parameters.11 Lack of standard scores or percentile rankings for an individual fetal Doppler value at a specific GA detracts substantially from the validity of a comparison of Doppler figures among fetuses with different GAs.

This study aimed to establish comprehensive reference ranges for spectral Doppler parameters of the fetal main branch PA during the second half of pregnancy from a large sample size population and to develop a percentile ranking solution for pertinent and potential parameters.