Protocol - Birth Weight - Measured Weight at Birth

Several overarching, critical issues for high-quality data collection of anthropometric measures that optimize the data in gene-environment etiologic research include: (1) the need for training (and retraining) of study staff in anthropometric data collection; (2) duplicate collection of measurements, especially under field conditions; (3) use of more than one person for proper collection of measurements where required; (4) accurate recording of the protocols and the measurement units of data collection; and (5) use of required and properly calibrated equipment.

Interpretation

For any measurement taken during pregnancy, maternal or fetal complication recorded, or measurement relating to a pregnancy outcome at delivery, such as maternal weight, gestational diabetes, preeclampsia, preterm delivery, or birth weight, the Working Group strongly advises collecting and recording a measure of gestational age at diagnosis and/or delivery. This entails recording the following from the medical records: (1) the gestational age in weeks at which the measurement or diagnosis was made, or the gestational age at delivery; (2) the type of gestational age (e.g., menstrual, conception, embryonic), although "gestational age" technically denotes menstrual age only; and (3) the source of the gestational age estimate, such as the recalled date of the mother’s last normal menstrual period (LMP), date of embryo transfer, fetal biometry from a first or second trimester ultrasound, fundal height, or neonatal physical examination. If linking with electronic vital records, care should be taken to research the history and definition of gestational age before use because criteria are likely to have evolved over time, and gestational ages estimated from different sources can significantly affect incidence and prevalence rates of important prenatal and perinatal categorizations, such as preterm delivery and small (SGA) and large for gestational age (LGA). Alternatively, depending on the source, the components for calculating gestational age could be recorded for estimating gestational age later, if necessary, although a clinician’s estimate of gestational age from a medical chart or vital records is preferred.

The presumed normal or optimal length of gestation, from the LMP date to the estimated date of delivery (EDD; sometimes denoted as estimated date of confinement, EDC), is 280 days (40 weeks) and 266 days from date of conception. Gestational age is generally recorded in medical charts in completed weeks; for example, a gestational age of 36 weeks represents the interval of 36 weeks, 0 days (written 36+0) to 36 weeks, 6 days (36+6). A pregnancy delivered before 37 completed weeks is considered preterm ("too early") and after 42 completed weeks is considered post-term ("too late"). By medical history, clinicians will first date a pregnancy using Naegele’s rule, which is done by calculating the EDD by counting back 3 months from the LMP and adding 7 days. This rule assumes that the gravida has a 28-day menstrual cycle with fertilization occurring on day 14. However, if the LMP is unknown, uncertain, or inconsistent with maternal symptoms, fetal size, and/or maturation, gestational age may be estimated by imaging (obstetric ultrasound) in the first trimester or up to 20 weeks. Afterward, the estimation may be by maternal examination, measurement, or examination of the fetus by ultrasound, or a physical examination of the neonate. Because there is greater variation in fetal size and timing of maturation as gestation progresses, EDD and gestational age estimates made early in gestation (first trimester) are more reliable and preferred.

Conventional indices derived using birth weight, in the absence of information on gestational age at delivery, include low birth weight (LBW; fewer than 2,500 g), very low birth weight (VLBW; fewer than 1,500 g), extremely low birth weight (ELBW; fewer than 1,000 g or fewer than 500 g), and high birth weight (HBW; greater than or equal to 4,000 g or greater than or equal to 4,500 g). About two-thirds of LBW infants are preterm by dates, and all term LBW infants are considered to be growth restricted. HBW can be termed "macrosomia," but that term is usually reserved to refer specifically to infants of diabetic mothers. If gestational age is available, the preferred indices are SGA, or small for dates, which is defined variously as less than 2nd, 3rd, 5th, or 10th percentile of a birth weight for gestational age reference, and LGA, or large for dates, which is defined as greater than or equal to 98th, 97th, 95th, or 90th percentile. The "normal" comparison within the percentile cutoffs is appropriate for gestational age. There is no universal birth weight for gestational age reference or standard for defining SGA and LGA, so the Working Group advises that each study operationally define these indices based on a thoughtful consideration of the available local, national, or international references. To index proportionality, neonates can be described as asymmetrically (wasted, less than -2 standard deviations weight for length or ponderal index) or symmetrically (stunted, less than -2 standard deviations length for gestational age, with a normal ponderal index) growth restricted.

For infants measured at birth, use the protocol titled Measured Weight at Birth (Global Network for Women’s and Children’s Health Research). For all others, the preferred method is abstraction from the birth certificate or medical record (protocol titled Measurement: Birth Weight Abstracted from Medical Records [National Vital Statistics System]). If this is not available, the self- or proxy-reported birth weight can be used (protocol titled Question: Proxy-Reported Birth Weight [National Health and Nutrition Examination Survey]). In all instances, the investigator should record the specific data source and specific protocol used.