As a first step, rates of adverse outcomes per 1000 live births were calculated according to infant sex, GA, and the studied period. Thereafter, for each time period, using the gestational age categories as the exposure variable (early term and late preterm infants, respectively, with midterm infants as the reference category), we used logistic regression to estimate odds for prematurity-related adverse outcomes such as neonatal death, low Apgar score (<7 at 5 min), pneumothorax, RDS, other respiratory conditions, and hyperbilirubinemia. Then, for each time period, odds ratios (ORs) with 95 % confidence intervals (CIs) for adverse prematurity-related outcomes among male infants, always in relation to their female counterparts, were calculated with logistic regression, separately for early term and for late preterm infants. Finally, to estimate the change in male risk before and after the introduction of ultrasound pregnancy dating, the relative change in odds for adverse prematurity-related outcomes between the two periods, among male infants in the early term and late preterm groups in relation to their female counterparts, was calculated as the ratio between the ORs for each period (Cohort Ratio, CR). The multivariate analyses were adjusted for maternal age, parity, and level of hospital care.
Brand new mathematical analyses was in fact conducted utilizing the SAS nine.step 3 software package (SAS Institute, Cary, NC, USA). The ORs and CRs through its 95 % believe durations was in fact projected into GENMOD procedure.
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The characteristics of your data cohorts are displayed during the Dining table step one , for the amount of female and male babies produced within the for every GA group throughout 1973–1978 and you may 1995–2010. The new proportion from midterm births (74.6 % in the early against. 70.5 % in the late months) try highest during the early cohort, and also the proportion regarding very early label births (21.0 % in the early versus. 25.5 % in the late months) is low in the first cohort.
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