Postnatal Estimates of Maturity

In the 1960s to 1970s, missing or inaccurate gestational age data for many newborns led to a search for postnatal methods of determining gestational age. These methods invariably focused on the degree of infant maturation (Allen, 2005a; Philip et al., 2003). Farr et al. (1966) described the maturation of a number of external physical characteristics in preterm and term infants. Hittner et al. (1977, 1981) proposed a systematic method of grading the disappearance of the pupillary membrane (i.e., the anterior vascular capsule of the lens of a preterm infant's eye) at 2-week intervals from 27 to 34 weeks of gestation. French investigators developed a method of measuring neurological maturity from observations of the changes in neck, trunk, and extremity flexor tone and posture with gestational age (Amiel-Tison, 1968; Amiel-Tison et al., 2002; Philip et al., 2003; Saint-Anne Dargassies, 1977). A number of postnatal clinical measures of the degree of maturation of external physical characteristics or neurological muscle tone, or both, were developed to estimate gestational age of preterm and fullterm infants (Allen, 2005a; Ballard et al., 1979, 1991; Dubowitz et al., 1970; Parkin et al., 1976).

These clinical postnatal measures are less predictive of gestational age (i.e. pregnancy duration at birth) at the extremes of gestation (i.e., in preterm and postterm infants) and in very sick infants (Alexander et al., 1990; Sanders et al., 1991; Shukla et al., 1987; Spinnato et al., 1984). The most widely used postnatal measures, the New Ballard Score (Ballard et al., 1991) and the Dubowitz gestational age assessment (Dubowitz et al., 1970), overestimate gestational age by 2 or more weeks in 45 to 75 percent of preterm infants with birth weights less than 1,500 grams (Sanders et al., 1991; Shukla et al., 1987; Spinnato et al., 1984). The accuracies of these measures decrease with an increase in gestational age (Alexander et al., 1990; Sanders et al., 1991). Gestational age is more often overestimated in African-American preterm infants than in white preterm infants, even when LMP, sociodemographic variables, pregnancy complications, and delivery characteristics are controlled for (Alexander et al., 1992).

Although some of these postnatal gestational age measures are extensively used to estimate gestational age at birth, first- and second-trimester ultrasounds are far more accurate at estimating gestational age (Alexander et al., 1990, 1992; Mitchell, 1979; Wariyar et al., 1997). In a comparison study, Wariyar et al. (1997) found that ultrasound before 20 weeks of gestation was the most accurate (95 percent confidence interval = +9 days, whereas the 95 percent confidence interval = +17 days for postnatal methods). For preterm infants with gestational ages of less than 30 weeks, an ultrasound performed before 20 weeks of gestation was more accurate than an ultrasound performed at or after 20 weeks of gestation for determination of gestational age at birth (95 percent confidence intervals = +9 days and +15 days, respectively), the New Ballard Score (95 percent confidence interval = +24 days), the Dubowitz gestational age assessment (95 percent confidence interval = +34), and the Parkin gestational age assessment (95 percent confidence interval = +26 percent).

The difficulty of using postnatal measures of degree of maturation of external physical characteristics and neurological muscle tone to estimate gestational age at birth highlights the difference between pregnancy duration and degree of maturation (Allen, 2005). Although conceptually the use of the words "gestational age" implies a time interval, duration of pregnancy, the measurement of gestational age has historically involved either measures of fetal or infant size, or measures of degree of infant maturation. Since degree of fetal maturation plays an impor tant role in infant mortality and morbidity rates, and may play a role in the signaling mechanisms for the normal initiation of labor at term, clarity in how gestational age is defined and determined is essential for understanding the mechanisms leading to preterm birth.

Beat Depression Now

Beat Depression Now

Let me be up front. My intention is to sell you something. Normally, it's not wise to come out and say that. However, I can do so because I have such an incredible deal for you that you'd be crazy to pass on it.

Get My Free Ebook

Post a comment