Obstetric dating and assessment
This cohort study included data on 1,314,602 births recorded in the Swedish Medical Birth Register.
We compared rates of prematurity-related adverse outcomes in male infants born early term (gestational week 37–38) or late preterm (gestational week 35–36), in relation to female infants, between a time period when pregnancy dating was based on the last menstrual period (1973–1978), and a time period when ultrasound was used for pregnancy dating (1995–2010), in order to assess the method’s influence on outcome by fetal sex.
In a study by Skalkidou et al., increased mortality and morbidity in post-term female infants in relation to male infants was seen after US was introduced in Sweden as the method for assessing GA .
Diagnoses are classified and recorded by the treating physician or midwife according to the International Classification of Diseases (ICD).
We compared prevalence of outcomes between sexes in a time period when LMP was used as the only method for dating pregnancies (1973–1978) and similarly the prevalence of outcomes were compared between sexes after US was introduced as the method for dating pregnancies (1995–2010).
The risk estimates, by fetal sex, that were generated for each of the two time periods were then compared.
As expected, adverse outcomes were lower in the later time period, but the reduction in prematurity-related morbidity was less marked for male than for female infants.
After changing the pregnancy dating method, male infants born early term had, in relation to female infants, higher odds for pneumothorax (Cohort ratio [CR] 2.05; 95 % confidence interval [CI] 1.33–3.16), respiratory distress syndrome of the newborn (CR 1.99; 95 % CI 1.33–2.98), low Apgar score (CR 1.26; 5 % CI 1.08–1.47), and hyperbilirubinemia (CR 1.12; 95 % CI 1.06–1.19), when outcome was compared between the two time periods.
Although often treated as term, late preterm infants more commonly present with prematurity-related morbidity such as hyperbilirubinemia, respiratory distress syndrome (RDS) of the newborn, transient tachypnea of the newborn, interventions to support breathing, and readmissions for hospital care .