Dating pregnancy ultrasound vs lmp
The risk estimates, by fetal sex, that were generated for each of the two time periods were then compared.
Information was collected from the Swedish Medical Birth Register (MBR), which contains information on more than 99 % of all births in Sweden since 1973.
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 other countries, the estimation is based on the date of the LMP or a combination of both methods.Early differences in fetal growth do exist [5–7], but it has been assumed that individual variation is too small to have any clinical significance . For example, second-trimester US dating underestimates GA and overestimates preterm delivery rates in infants born small for gestational age (SGA) .Furthermore, postponing the date of estimated delivery by 7 days by US is associated with birth of SGA infants , increased risk of stillbirth, low Apgar score, and neonatal death .By contrast, from 1995 on, nearly all clinics based EDD assessment on US biometry and documented the EDD-US in the MBR.Routine US scanning has been offered to all pregnant women since 1990, and more than 95 % of the women accept this offer .
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Our hypothesis was that a change in the method of dating pregnancy might have led to an increased risk for prematurity-related adverse outcomes among male infants in relation to female infants by introducing a misclassification bias due to sex differences in early growth.