Gestational age can be determined prenatally by the following techniques: date of last menstrual period, date of first reported fetal activity (quickening usually occurs at 16–18 weeks), first reported heart sounds (10–12 weeks by Doppler ultrasound examination), and ultrasound examination (very accurate if obtained before 20 weeks' gestation). The most reliable measure of gestational age is done prenatally during the first trimester. Postnatal gestational age assessment is unreliable in infants that are extremely premature and postterm. The American Academy of Pediatrics recommends that all newborns be classified by birthweight and gestational age. The most common techniques for determining gestational age in the immediate postnatal period are discussed in this chapter.
Classification. Infants are classified as preterm (<37 weeks), term (37–41 6/7 weeks), or postterm (≥42 weeks). Refinements developed in neonatal assessment have provided additional classifications based on a combination of features.
Small for gestational age (SGA) is defined as 2 standard deviations below the mean weight for gestational age or below the 10th percentile (see Appendix E). (For a full discussion, see Chapter 97.) SGA is commonly seen in infants of mothers who have hypertension or preeclampsia or who smoke. This condition is associated with maternal factors (chronic disease, malnutrition, conditions affecting the blood flow and oxygenation in the placenta), placental factors (infarction, previa, abruption, anatomic malformations, etc.) and fetal factors (congenital infections [TORCH; see Chapter 127], chromosomal abnormalities, dysmorphic syndromes, and other congenital anomalies).
Appropriate for gestational age (AGA). See Appendix E.
Large for gestational age (LGA) is defined as 2 standard deviations above the mean weight for gestational age or above the 90th percentile (see Appendix E). LGA can be seen in infants of diabetic mothers (see Chapter 94), infants with Beckwith-Wiedemann syndrome and other syndromes, constitutionally large infants with large parents, or infants with hydrops fetalis.
Methods of determining postnatal gestational age
Rapid delivery room assessment. The most useful clinical signs in differentiating among premature, borderline mature, and full-term infants are (in order of usefulness): creases in the sole of the foot, size of the breast nodule, nature of the scalp hair, cartilaginous development of the earlobe, and scrotal rugae and testicular descent in males. These signs and findings are listed in Table 4–1, which enables one to make a rapid assessment at delivery.
New Ballard Score. The Ballard maturational score has been expanded and updated to include extremely premature infants. It has been renamed the New Ballard Score (NBS). The score now spans from 10 (correlating with 20 weeks' gestation) to 50 (correlating with 44 weeks' gestation). It is best performed at <12 h of age if the infant is <26 weeks' gestation. If the infant is >26 weeks' gestation, there is no optimal age of examination up to 96 h.
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