Chapter 6: Gestational Age and Birthweight Classification

INTRODUCTION

Gestation is the period of fetal development from the time of conception to birth. Gestational age (or menstrual age), as defined by the American Academy of Pediatrics (AAP), is the “time elapsed between the first day of the last menstrual period (LMP) and the day of delivery.” It is expressed in completed weeks (26-week and 4-day-old fetus is expressed as a 26-week fetus). Gestational age is important information for the obstetrician to provide obstetric care. It is critical information for the neonatologist for evaluation of the infant and to anticipate high-risk infants and complications. Gestational age and birthweight classification help the neonatologist to categorize infants, guide treatment, and assess risks for morbidity and mortality. Neonates can be classified based on gestational age (preterm, late preterm, early term, full or late term, post term), birthweight (eg, extremely low birthweight, very low birthweight, low birthweight), and gestational age and birthweight combined (small for gestational age [SGA], appropriate for gestational age [AGA], large for gestational age [LGA]). The AAP recommends that all newborns be classified by birthweight and gestational age.

I. GESTATIONAL AGE ASSESSMENT

Gestational age can be determined prenatally in the fetus and postnatally in the newborn. Gestational age is determined by the “best obstetric estimate,” which is based on 4 parameters: first day of LMP, physical examination of the mother, prenatal ultrasound, and history of assisted reproduction. The American College of Obstetricians and Gynecologists, the American Institute of Ultrasound in Medicine, and the Society for Maternal-Fetal Medicine indicate that antenatal first-trimester ultrasonography is the most accurate method to establish gestational age in a pregnancy not achieved through assisted reproductive technologies. The methods they recommend in the first trimester (ultrasound, LMP data) represent the best obstetric estimate for clinical care, and this date should be recorded on the birth certificate. Some feel that the most accurate estimation of gestational age is based on the prenatal estimation (using ultrasound and LMP) in combination with the postnatal assessment using physical and neurologic maturity. Gestational age can also be determined postanally using various examinations including assessment of physical and neuromuscular maturity. It is done if the best obstetric estimate is inaccurate. Postnatal gestational assessment is important, especially in cases where there is no prenatal care, LMP is unknown, and no ultrasounds were done.

1. Prenatal gestational age assessment. Determined by a combination of date of the LMP and prenatal ultrasound examination. Based on these, the obstetrician is able to give his or her “best estimate” of gestational age, since variability up to 2 weeks can occur. Use of quickening and physical examination of the mother are not very accurate and are not used if ultrasound is available.

   1. Maternal history

      1. First day of the last menstrual period (LMP). This clinical estimate is very reliable if dates are remembered but depends on an accurate menstrual history and normal maternal physiology. The first day of the LMP is about ...