Abstract
BackgroundAssessing gestational age by ultrasound can introduce a systematic bias due to sex differences in early growth.MethodsThis 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.ResultsAs 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. A similar trend was seen for late preterm male infants.ConclusionMisclassification of gestational age by ultrasound, due to size differences, can partially explain currently reported sex differences in early term and late preterm infants’ adverse neonatal outcomes, and should be taken into account in clinical decisions and when interpreting study results related to fetal sex.Electronic supplementary materialThe online version of this article (doi:10.1186/s12884-016-1129-z) contains supplementary material, which is available to authorized users.
Highlights
Assessing gestational age by ultrasound can introduce a systematic bias due to sex differences in early growth
The aim of this study was to compare rates of adverse prematurity-related outcomes in early term and late preterm male infants in relation to their female counterparts, between a time period, when pregnancy dating was based on the last menstrual period (LMP), and a time period when ultrasound was used for pregnancy dating, in order to assess the dating method’s influence on prematurity-related adverse outcomes by fetal sex
Rates of adverse outcomes were lower for female infants than for male infants, in both time periods, with the exception of neonatal death among late preterm infants in the later period
Summary
Assessing gestational age by ultrasound can introduce a systematic bias due to sex differences in early growth. Biometry using US in the first or second trimester is generally a more precise method for assessing GA and pregnancy dating, than estimates based on the date of the LMP [2]. Differences in fetal growth do exist [5,6,7], but it has been assumed that individual variation is too small to have any clinical significance [8]. Recent studies challenge this assumption [9,10,11,12]. Postponing the date of estimated delivery by 7 days by US is Kullinger et al BMC Pregnancy and Childbirth (2016) 16:335 associated with birth of SGA infants [11], increased risk of stillbirth, low Apgar score, and neonatal death [10]
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