Table 1 Overview of definitions and measurements relating to gestational age
Gestational age | Measure of pregnancy duration in weeks, from the first day of the woman’s last menstrual cycle to the date of assessment |
|---|---|
Preterm birth | Preterm birth is defined by WHO as ‘any birth before 37 completed weeks of gestation, or fewer than 259 days since the first day of the women’s last menstrual period (LMP)’ [4, 5]. It is subdivided into extremely preterm (< 28 weeks); very preterm (28-< 32 weeks) and moderate or late preterm (32-< 37 weeks) [6]. |
Post-term birth | Post-term birth is defined by WHO as ‘any birth at/after 42 completed weeks of gestation since the first day of the women’s last menstrual period (LMP)’ [4, 5]. |
Measuring gestational age before birth | Ultrasound (gold standard): Accurate to ± 5 to ± 21 days depending on the timing of ultrasound at different stage of pregnancy and biometric parameters (crown-rump length, biparietal diameter, femur length, abdominal circumference, head circumference). CRL was used for pregnancy of < 14 weeks, and BPD and FL were used for ≥ 14 to < 24 weeks. Last menstrual period (LMP): Women’s recall of the date of the first day of her last period. Accurate to ~ ± 14-21 days. Affected by variation in menstrual cycles (e.g. with under-nutrition or after cessation of hormonal contraceptive methods, or breastfeeding), socio-cultural attitudes to menstruation, literacy levels and digit preference. Best obstetric measure: Combination of ultrasound and LMP. Fundal height: Symphysis-fundal height measurement is another method that can estimate gestational age before birth. WHO guideline development group does not recommend this method due to lack of evidence in accuracy [7]. |
Measuring gestational age after birth | There are a range of clinical assessments, e.g. Dubowitz, Ballard, Finnstrom scores and others [8, 9]. This study does not apply these methods. |