Gestational age and growth parameters (see Growth Parameters in Neonates) help identify the risk of neonatal pathology. Gestational age (menstrual age, postmenstrual age) is the time elapsed since the beginning of the woman's last menstrual period; it is usually counted in weeks. Because it is not based on the moment of fertilization, which is difficult to specify (except when in vitro fertilization is done), gestational age is not the actual age of the fetus. Gestational age is the primary determinant of organ maturity.
The best way to assess gestational age is with antenatal ultrasonography and menstrual history. Clinicians also estimate gestational age during the newborn physical examination (see The First Few Hours) using the new Ballard score (see Figure: Assessment of gestational age—new Ballard score.). The Ballard score is based on the neonate's physical and neuromuscular maturity and can be used up to 4 days after birth (in practice, the Ballard score is usually used in the first 24 h) . The neuromuscular components are more consistent over time because the physical components mature quickly after birth. However, the neuromuscular components can be affected by illness and drugs (eg, Mg sulfate given during labor).
Assessment of gestational age—new Ballard score.
Based on gestational age, each neonate is classified as