Postnatal assessment of gestational age

INTRODUCTION — The most reliable method of determining gestational age (GA) is the knowing with certainty the date of last menstrual period (LMP) or in-vitro fertilization (calculator 1). Estimations from antenatal ultrasound performed at <22 weeks gestation are also reliable. It can also be helpful to postnatally assess GA, particularly if a reliable estimation from one of these methods is not available or if there is a discrepancy between the prenatally estimated GA and the appearance of the newborn. GA can be estimated postnatally based on neonatal physical examination and neuromuscular assessment.

This topic will discuss the postnatal assessment of GA. Prenatal assessment of GA is discussed separately. (See "Prenatal assessment of gestational age, date of delivery, and fetal weight".)

METHODS OF POSTNATAL ASSESSMENT — The principal method used to postnatally estimate gestational age (GA) is the new Ballard score, which combines physical and neurologic criteria (table 1 and figure 1). The new Ballard score is easier to administer than the longer Dubowitz method. (See 'Ballard score' below and 'Dubowitz method' below.)

Examination of the anterior capsule of the lens of the eye (figure 2) and electroencephalography may provide additional information if there is ongoing uncertainty. (See 'Eye examination' below and 'Electroencephalography' below.)

The physical indicators of maturity, which can be evaluated immediately after delivery, include (table 1):

●Appearance of the skin

●Plantar creases

●Appearance of the breast

●Lanugo

●Characteristics of the eyelids and ear cartilage

●Appearance of the genitalia

For the purposes of GA estimation, the neuromuscular examination focuses on assessment of [1]:

●Posture

●Active and passive tone

●Reflexes

An accurate neuromuscular examination requires that the infant is in an alert and rested state, which may not occur during the first day, especially if the infant is ill or under the influence of maternal medications. Thus, for newborns ≥26 weeks, the examination is most reliable when performed at 30 to 42 hours of age. However, for newborns <26 weeks, it is best to perform the assessment prior to 12 hours of age since determination of the GA influences clinical decisions. (See "Periviable birth (limit of viability)", section on 'Postnatal management'.)

Ballard score — The new Ballard score is the main method used to postnatally estimate GA (table 1 and figure 1). This score was developed by modifying the original Ballad score to improve assessment of infants as preterm as 20 weeks [2].

●Original Ballard score — The Ballard score is based upon six physical and six neurologic criteria [3]. It is a relatively simple assessment that can be accomplished quickly (usually in <5 minutes) [3].

The main limitation of the original Ballard score is that it performed poorly in preterm, post-term, and small for gestational age (SGA) newborns. As an example, in a comparison with assessment by early ultrasonography, the Ballard examination underestimated the GA in approximately 75 percent of post-term newborns and it overestimated GA in approximately 50 percent of very preterm neonates [4].

●New Ballard score – The new Ballard score includes expanded descriptions of the six physical and six neurologic features in the original Ballard score so that they can be applied to newborns from 20 to 44 weeks gestation. The scores of each feature are added to calculate a maturity rating that correlates with gestational age (figure 1 and table 1). The new Ballard score is accurate within one to two weeks [2,5,6]. For newborns ≥26 weeks, the examination is most reliable when performed at 30 to 42 hours of age; for newborns <26 weeks, it is best if done prior to 12 hours of age [2].

In the initial report, the new Ballard score slightly overestimated GA compared with estimation by last menstrual period (LMP) and/or ultrasound (by 0.15 weeks for the entire cohort and by 0.32 weeks for newborns <26 weeks GA) [2]. Correlation was similar when the examination was performed up to 96 hours of age.

However, in a subsequent study that included 223 newborns from 22 to 28 weeks GA (as determined by accurate menstrual history), the new Ballard score overestimated GA by 1.3 to 3.3 weeks [5]. Variation in fetal maturation probably accounts for the inaccuracy of the Ballard examination and other estimates based on physical or neurologic characteristics.

A systematic review identified 30 studies evaluating the accuracy of the original Ballard score (20 studies) and new Ballard score (10 studies) [6]. Compared with determining GA by LMP, the Ballard score slightly overestimated GA (mean difference 0.7 weeks; 95% CI 0.36-1.04 weeks). The Ballard score more closely approximated GA based on ultrasound/best obstetric estimate, though it still slightly overestimated GA (mean difference 0.4 weeks; 95% CI 0-0.81 weeks).

Dubowitz method — The Dubowitz method was widely used before the development of the new Ballard score. The revised Dubowitz scoring system incorporates 34 physical and neurologic assessments [7,8]. These are divided into six categories (tone, tone patterns, reflexes, movements, abnormal signs, and behaviors), and each are assigned scores based on an instructions sheet with illustrations. Higher scores indicate greater maturity. The scores are added, and the total score is plotted on a graph to estimate gestational age. The physical features allow differentiation of gestational ages in infants greater than 34 weeks. The neurologic criteria are important between 26 and 34 weeks, when physical differences are less apparent.

However, the Dubowitz system has two important disadvantages:

●The main disadvantage is that it includes a large number of criteria and therefor requires a substantial amount of time (approximately 15 to 20 minutes) to complete the evaluation. This is particularly impractical when evaluating acutely ill or extremely preterm newborns. As discussed above, the Ballard system has replaced the Dubowitz method as the standard assessment because it is simpler and quicker with similar accuracy. (See 'Ballard score' above.)

●The other disadvantage is that it tends to overestimate GA in preterm infants (as does the Ballard score). As an example, in a study of 110 preterm infants (mean GA 28.3 weeks based on LMP and best obstetric estimate), the Dubowitz examination overestimated the GA on average by 2.8 weeks [9]. A systematic review identified 26 studies evaluating the accuracy of the Dubowitz method in term and preterm newborns [6]. Compared with determining GA by LMP, the Dubowitz method slightly overestimated GA (mean difference 0.65 weeks; 95% CI 0.01-1.30 weeks). However, the Dubowitz method closely approximated the GA based on ultrasound/best obstetric estimate (mean difference 0.2 weeks) (95% CI -0.51 to +0.55 weeks).

Rapid assessment of gestational age — Some studies have shown that using a few, select physical or neurologic elements of the Ballard (or Dubowitz) method correlates well with LMP, early ultrasound, or the full Dubowitz/Ballard exam for quick estimation of gestational age [10,11]. This technique has also proven beneficial when less experienced health care providers are tasked with estimating GA [12].

A systematic review identified 12 studies evaluating the accuracy of various individual physical criteria for estimating GA [6]. The physical characteristics that were most accurate for this purpose were breast size, plantar skin creases, ear firmness, and skin texture. The same systematic review identified 10 studies evaluating the accuracy of various individual neuromuscular signs for estimating GA [6]. The most informative signs were posture, ventral suspension, and square window.

When performing assessments of GA based on only a few physical and/or neurologic criteria, clinicians must be aware of the following pitfalls:

●Impact of maternal medications – Neuromuscular findings in the newborn can be affected by certain maternal medications during labor, such as magnesium sulfate or narcotics.

●Neurologic impairment – Newborns with perinatal asphyxia or those with neuromuscular diseases may have neurologic findings that misrepresent their actual GA. (See "Perinatal asphyxia in term and late preterm infants".)

●Fetal growth restriction (FGR) – Newborns with FGR can have increased desquamation and wrinkling of the soles of the feet, giving a more mature appearance. Indicators that are most reliable when assessing GA in newborns with FGR include the neuromuscular assessment and the eye examination. (See "Fetal growth restriction (FGR) and small for gestational age (SGA) newborns", section on 'Postnatal diagnosis'.)

Eye examination — The disappearance of the anterior vascular capsule of the lens occurs in an orderly sequence between 27- and 34-weeks gestation. As a result, examination with a direct ophthalmoscope after dilation of the pupil can be used to estimate gestational age [13]. The disappearance of the vascular system is divided into four grades that correlate with gestational age (figure 2). The correlation between the grade and gestational age is highly significant and is independent of intrauterine growth restriction [13,14]. This method cannot be used before 27 weeks gestation because the cornea is too opaque to permit adequate visualization; after 34 weeks gestation, the vessels usually are completely resorbed. Because the vascular system atrophies rapidly after birth, the examination should be performed before 48 hours of age.

Electroencephalography — A characteristic developmental sequence of electroencephalographic (EEG) patterns occurs with increasing postconceptual age, beginning at 21 to 22 weeks [15-18]. As an example, in a study of infants born at less than 27 weeks gestation, continuous EEG patterns increased, and immature EEG findings (eg, interburst intervals and discontinuous patterns) decreased with increasing age [18]. Although this method is not used clinically, patterns of regional and hemispheric electrical activity that appear at specific times during maturation can be used to estimate gestational age [19].

EEG and ultrasound or anatomic assessment of gestational age are highly correlated [20-22]. In infants less than 30 weeks gestation who subsequently died, the EEG correlated better than did the Ballard score with neuropathologic assessment of sulcal-gyral development (92 versus 74 percent agreement) [20]. The maturational progression of EEG background activity occurs at the same rate in the fetus or preterm infant [19].

SUMMARY

●Prenatal assessment of GA ‒ The most reliable method of determining gestational age (GA) is the knowing with certainty the date of last menstrual period (LMP) or in-vitro fertilization (calculator 1). Estimations from antenatal ultrasound performed at <22 weeks gestation are also reliable. (See "Prenatal assessment of gestational age, date of delivery, and fetal weight".)

●Postnatal assessment of GA ‒ Postnatal assessment of GA is based on physical examination and neuromuscular assessment. The new Ballard score is the main tool used to postnatally estimate GA (table 1 and figure 1). It is a relatively simple assessment that can be accomplished quickly (usually in <5 minutes) and is accurate within one to two weeks. Other methods that are used less commonly include the Dubowitz score, examination of the anterior capsule of the lens of the eye (figure 2), and electroencephalography. (See 'Ballard score' above and 'Methods of postnatal assessment' above.)

•Physical indicators of maturity – The physical indicators of maturity include (table 1):

-Appearance of the skin

-Plantar creases

-Appearance of the breast

-Lanugo

-Characteristics of the eyelids and ear cartilage

-Appearance of the genitalia  

•Neuromuscular indicators of maturity – Neuromuscular indicators of maturity include (figure 1):

-Posture

-Active and passive tone

-Reflexes

•Timing of assessment – The physical indicators of maturity can be assessed immediately after birth. However, an accurate neuromuscular examination requires that the infant is in an alert and rested state, which may not occur during the first day, especially if the infant is ill or under the influence of maternal medications. Thus, for newborns ≥26 weeks, the examination is most reliable when performed at 30 to 42 hours of age. However, for newborns <26 weeks, it is best to perform the assessment prior to 12 hours of age since determination of the GA influences clinical decisions. (See 'Ballard score' above and "Periviable birth (limit of viability)", section on 'Postnatal management'.)