Ultrasound dating of pregnancy

In Sweden, a second trimester scan, using the biparietal diameter (BPD)-measurement for pregnancy dating, is typically performed around gestational week 18 at what time the mean difference (male vs. An introduced bias in the GA estimate, due to size difference by fetal sex at the time of pregnancy dating, would be hypothesized to affect clinical management and neonatal outcomes in the late preterm and early term period.

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 [13].

Such routine scanning is typically performed early in the second trimester (gestational weeks 17–19) for assessing GA, detection of multiple births, placental location, and congenital anomalies [1].

According to a 1996 study, when 59 clinics in Sweden provided obstetric and antenatal care, US scanning was performed at gestational weeks 16–20 in 52 clinics and at 10–15 weeks in three clinics [1].

The version used during 1973–1978 was ICD-8, while during 1995–2010 the versions were ICD-9 and ICD-10.

Up to 1978, practically all clinics were using the LMP date in order to calculate the EDD.

A similar trend was seen for late preterm male infants.

Since then, the use of first-trimester US has increased gradually.

Our study population consisted of two cohorts including all singleton births in Sweden, with valid birth dates for both mother and infant, from 1973 to 1978 (GA assessment based on the LMP date) and from 1995 to 2010 (GA assessment by US).

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.

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.

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Assessing gestational age by ultrasound can introduce a systematic bias due to sex differences in early growth.

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