Gestational age of a fetus or of a newborn can be assessed currently by three different methods: the mother's menstrual history, prenatal ultrasonography and the postnatal maturational examination.
An accurate menstrual history, when obtainable, remains the best measure of gestational age, but depends upon normal maternal physiology and an accurate and reliable history.
The prenatal ultrasound examination is one indirect method of assessing the gestational age of the fetus. Based upon fetal body part measurements, it relies upon normally timed and proportioned fetal growth rates. When performed early in gestation, fetal ultrasound is a highly accurate method of assessing gestational age. As the conceptus is exposed to a variety of intrauterine influences, fetal growth may be affected in a variety of ways. (ref 1) Fetal ultrasound measurements, affected by differential fetal growth, become increasingly subject to the intrauterine environment as a gestation progresses. Late trimester ultrasound measurements are therefore fraught with unavoidable inaccuracies as indicators of fetal gestational age. (ref 2,3)
The maturational examination, a postnatal, indirect method of assessing gestational age, is based upon indicators of fetal neuromuscular and physical maturation. (ref 4) As with fetal growth, fetal maturation may be influenced by a variety of intrauterine experiences. Stressful fetal experiences may accelerate pulmonary (ref 5) and neuromuscular (ref 6) rates of maturation while slowing or not affecting physical maturation. A completely non-stressed fetus may mature more slowly than the average fetus. The same events that accelerate fetal maturation may adversely affect fetal growth. Conversely, those that accelerate fetal growth may delay its maturation.