Pregnancy dating

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Although the US-based method is superior to the LMP-based method in most pregnancies, some maternal and fetal characteristics, such as the sex of the fetus, may influence the precision of the US-based estimate, and this lack of precision may be associated with adverse perinatal outcomes.The discrepancy between dating methods and its association with pregnancy-related outcomes has been investigated in a few studies, but these have included a small study size or a limited number of perinatal outcomes.During the study period, US scanning was offered to all pregnant women and was accepted by .According to a 1996 study of the 59 clinics in Sweden that provided obstetric and antenatal care, pregnancy dating was based on a routine US examination performed between gestational weeks 16–20 in 52 clinics, and on a US examination performed at 10–15 weeks in three clinics.

A negative discrepancy between dating methods was associated with lower odds than expected for all adverse delivery outcomes related to large infants, except for shoulder dystocia.The reference category was defined as a discrepancy within 2 days of the median. Delivery outcomes were included if adverse outcomes were expected to be more frequent among large infants at birth because a larger fetal size may be apparent at the time of the dating scan: prolonged second stage of labor (6622, O63.1), delivery by forceps or vacuum extractor (6695, O81), delivery by cesarean section (6697, O82), shoulder dystocia (6604, O66.0), postpartum hemorrhage (666, O72), and perineal laceration: third or fourth degree (6642, 6643, O70.2, O70.3) (Supplementary Table 1).The remaining pregnancies were defined as a small negative or small positive discrepancy (Fig. To check for any association with the discrepancy between dating methods to infant size at birth, we included small for gestational age (SGA) and large for gestational age (LGA).A sensitivity analysis was performed in the crude model and in models 1–3 after exclusion of SGA and LGA births.Numbers of events, event rates, risk differences (the difference in event rate between large discrepancy and reference categories), and numbers needed to treat (NNT = 1/risk difference) were calculated for the two large discrepancy categories in relation to the reference category.

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