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.
New statistical analyses was in fact conducted utilising the SAS 9.step 3 software package (SAS Institute, Cary, NC, USA). The latest ORs and you will CRs and their 95 % count on menstruation was estimated to your GENMOD techniques.
Efficiency
The features of the analysis cohorts is actually showed within the Dining table 1 , into the level of male and female babies produced from inside the per GA classification during the 1973–1978 and you can 1995–2010. The brand new proportion of midterm births (74.6 % in early vs. 70.5 % on later several months) is high in the early cohort, additionally the proportion out of early term births (21.0 % during the early vs. 25.5 % from the late several months) was low in early cohort. New ratio recently preterm births are equivalent in the several cohorts (cuatro.3 % during the early versus. 4.0 % on later period). Indicate maternal decades for the first time period (1973–1978) are 27 decades (SD = 5), and you may mean maternal ages regarding the later time period (1995–2010) was 31 (SD = 5).
Dining table step one
Amount and you will shipment regarding two cohorts of singleton births when you look at the Sweden, that have indicate and you will standard deviation (SD) out-of birthweight, beginning duration, and you can Apgar rating of the kids sex and you may gestational many years, both before and after the newest implementation of ultrasound to possess maternity dating