The research implications of the selection of a gestational age estimation method

Abstract

There are three primary methods of gestational age estimation: dating based on last menstrual period (LMP), ultrasound-based dating and neonatal estimates. We review the strengths and limitations of each method as well as their implications for research. Dating based on LMP is a simple, low-cost method of estimating gestational age. Limitations associated with the use of menstrual-based dating include reporting problems such as uncertainty regarding the LMP date, possibly due to bleeding not associated with menses, as well as concerns about the incidence of delayed ovulation, which can result in invalid estimates of gestation, even for women with certain LMP dates. Given that most women in the US have at least one ultrasound during pregnancy, it is becoming increasingly common for clinicians to verify menstrual dates using early ultrasound. To calculate gestational age with the use of ultrasound, fetal measurements are compared with a gestational age-specific reference. The primary limitation of this method is the fact that the gestational age estimates of symmetrically large or small fetuses will be biased. Further, given that ultrasound references were developed using pregnancies that were dated according to reliable LMP dates, they are potentially biased in the same direction as dates calculated according to LMP. Neonatal estimates of gestational age have been shown to be the least precise dating method. To highlight the research implications of the choice of a gestational dating method, we used data from the Routine Antenatal Diagnostic Imaging with Ultrasound Study to identify risk factors for post-term delivery. Risk factors for post-term delivery are shown to vary according to the choice of a gestational dating method, suggesting that some findings are an artefact of the choice of a method rather than evidence of causality.

Approach to Infants Born at 22 to 24 Weeks’ Gestation: Relationship to Outcomes of More-Mature Infants

“…Although our study was limited by the uncertainties inherent in determining gestational age, the estimates used in our analyses are the same as those used by clinicians discussing potential outcomes of preterm infants with families. [17][18][19] Antenatal corticosteroid use is known to improve clinical outcomes in infants $26 weeks' gestation at birth. 20 In a recent analysis of NICHD NRN data, investigators observed a similar association between exposure to antenatal corticosteroid use among 22-to 25-week infants on the combined outcome of death or NDI (OR = 0.58 [95% CI: 0.42, 0.80]) 21 ; however, these results may be biased by the selective use of corticosteroids in pregnancies with other favorable prognostic factors.…”

Global health indicators and maternal health futures: The case of Intrauterine Growth Restriction

“…IUGR diagnoses are calculated using 280 days to ideal foetal maturation and the onset of spontaneous labour (Saunders & Paterson, 1991), but other studies determined full maturation occurs at 279 days (Kieler, Axelsson, Nilsson, & Waldenstro, 1995), 281 days (Bersjø, Denman, Hoffman, & Meirik, 1990) and even 288 days (Mittendorf, Williams, Berkey, & Cotter, 1990). Each gestational day adds grams (Lynch & Zhang, 2007), so the day convention chosen in the machine is also a factor in predictive accuracy.…”

New First-Trimester Crown-Rump Length Equations from a French General Population

“…A total of 513 pregnancies complied with our selection criteria and were scored. As estrus occurs 14 days before menstruation, and not in the middle of the menstrual cycle (due to delayed ovulation [13]), day 0 of fetal life was determined as 14 days before the next estimated menstruation. In case of a multiple pregnancy, the measurement of only 1 embryo was recorded.…”