Study design and population

A hospital-based prospective study was conducted for a period of one year (November 2020–2021) in a tertiary level Maternal and child care hospital of South India.

In our Institute, in addition to the routine third trimester ultrasound scan performed between 28 and 32 weeks, we also perform another routine third trimester scan between 34 and 39 weeks of gestation as an Institutional protocol. The inclusion criteria set in the study were all singleton pregnancies with gestational age (measured from dating by certain last menstrual period or by first-trimester USG) between 34 and 39 weeks and only foetuses delivered within 48 h of admission. Exclusion criteria in the study were patients with oligohydramnios (defined as Amniotic Fluid Index (AFI) less than 5 cm), polyhydramnios (defined as AFI more than 25 cm), maternal Gestational diabetes mellitus (GDM), Pregnancy-induced hypertension (PIH), maternal systemic corticosteroid exposure prior to scan, foetal Intra-Uterine Growth Restriction (IUGR), foetuses with structural abnormalities on USG or after delivery and foetuses delivered 48 h after USG. We evaluated a total of 729 pregnant women between 34 and 39 weeks of gestational age. We excluded 57 patients with oligohydramnios, 23 patients with polyhydramnios, 41 patients with GDM, 29 patients with PIH, 39 patients who had received corticosteroids prior to the scan, 54 foetuses with IUGR, 69 foetuses delivered more than 48 h after the USG, 45 foetuses with structural abnormalities on USG and 15 neonates with structural abnormalities diagnosed only after delivery leaving 357 foetuses. In 15 of those 357 foetuses, the MPA Doppler waveforms could not be elucidated accurately because either the foetal spine was directed anteriorly (8 foetuses) or the women were obese (7 women). Technically acceptable Doppler waveforms were obtained in the rest thus leaving 342 eligible foetuses for final analysis.

Late preterm, in our study, was defined as those infants delivered between 34 0/7 and 36 6/7 weeks of gestation and those delivered between 37 0/7 and 38 6/7 weeks of gestation were defined as early term neonates [9].

Ethical consent

The study was approved by the Institutional ethics review committee. All the participants were informed about the procedure and written and informed consent was obtained.

Imaging techniques

Ultrasound examinations were performed using SAMSUNG RS 80 EVO Ultrasound machine equipped with a 2–6 MHz (Mega Hertz) curvi-linear transducer. All the Doppler examinations were performed by a single Radiologist with 10 years of experience in the field of Obstetric Ultrasonography.

After a routine Antenatal third trimester USG, which included foetal biometry, Estimated Foetal Weight (EFW) and AFI, examination of the foetal heart was done systematically using the three-vessel view, 4 chamber view and outflow tracts. The foetal thorax was carefully analysed in axial view with the foetus at rest, the foetal MPA was followed till the middle of the pulmonary valve and its branching into right and left pulmonary arteries (Fig. 1A and B). The pulsed Doppler technical adjustments like keeping sample gate to about 3 mm and angle of insonation less than 60° were made. Doppler gain and scale were adjusted till Peak Systolic Velocity (PSV) and early diastolic notch were visualized optimally. The MPA Doppler waveform shape is commonly termed as ‘Spike and Dome’ pattern due to needle-like appearance caused by sharp systolic peak and it also produces a small notch of reversed flow at the end of systole. This waveform is distinctive and must not be confused with a rounded and triangular shaped pattern with greater diastolic flow of ductus arteriosus waveform [10].

Fig. 1
figure 1

Evaluation of the foetal main pulmonary artery (MPA) and measurement of Doppler indices. A sonographic short-axis view of normal foetal heart at 35 weeks and 3 days of gestational age. Right (R) and left (L) pulmonary arteries are shown branching from the Main pulmonary artery (MPA). Anteriorly and to the right of MPA are aorta and SVC. B Colour Doppler image in short-axis view of normal foetal heart at 35 weeks and 3 days of gestational age. C Foetal Main pulmonary artery velocity waveforms with measurement of Doppler indices in a foetus at 35 weeks and 6 days of gestational age. On post-natal follow-up, the neonate did not develop RDS. D Foetal Main pulmonary artery velocity waveforms with measurement of Doppler indices in a foetus at 35 weeks and 3 days of gestational age which subsequently developed RDS

After obtaining the optimal MPA waveform, Doppler velocity variables were manually measured thrice and its mean was calculated. At/Et ratio, RI, S/D ratio, PI and PSV were the variables taken into account (Fig. 1C and D). The At /Et ratio was obtained by dividing the time interval from the commencement of ventricular systole till attainment of peak velocity(At) by the time interval between the commencement till the end of ventricular systole (Et).

Diagnosis of neonatal RDS

A single paediatrician, who had no knowledge of the foetal MPA Doppler measurements handled the recording of the Neonatal birth weight (NBW) and Apgar scores at 1 min and 5 min. The mode of delivery as well as the neonatal gender was documented.

The diagnosis of RDS was made if any two of the three following criteria were met:

  1. (1)

    Respiratory failure (tachypnoea, retraction and/or nasal flaring) shortly after birth and increased oxygen requirement (fractional concentration of inspired oxygen > 0/4) for more than 24 h.

  2. (2)

    Radiographic findings of respiratory distress syndrome, such as bilateral diffuse ground glass opacity with air bronchogram in the absence of other respiratory ailments.

  3. (3)

    Response to exogenous pulmonary surfactant.

Statistical analysis

Student’s T Test was done for comparing independent samples and Chi Square test (χ2) for comparing categorical data. Pearson’s moment correlation equation was used to correlate between different variables. Receiver Operator Characteristic (ROC) analysis was used to predict neonatal RDS after deriving optimum cut-off value of At/Et ratio.

p values of < 0.05 was considered statistically significant. The collected data were entered in Microsoft Excel and analysed using SPSS version 24.0.

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