Postnatal assessment of gestational age

INTRODUCTION — Gestational age is determined in the fetus by menstrual history or ultrasound dating and in the newborn by physical examination and neuromuscular assessment. This determination is used to assess intrauterine growth, make decisions about the timing of delivery in complicated pregnancies, and predict the infant's clinical course. The most accurate estimation of gestational age is achieved by considering prenatal date (ie, last menstrual period [LMP] and early ultrasound [if available]) in combination with physical and neurologic features of the newborn.

DEFINITIONS

●Gestational age (also called menstrual age or menstrual dating) is estimated in completed weeks from the date of the mother's last menstrual period (LMP).

●Estimated date of a term delivery (also known as estimated date of confinement [EDC]) is calculated by adding one year and seven days to the LMP and then subtracting three months. This calculation is known as Naegele's rule and assumes that ovulation occurs two weeks after the LMP.

●Ovulatory or fertilization age of the fetus is the presumed date of ovulation and is two weeks less than gestational age measured from the LMP. Ovulatory age of the fetus is frequently used by embryologists.

OVERVIEW — Accurate menstrual dating depends upon certain maternal knowledge of last menstrual period (LMP) and regular 28-day cycle length and is considered to have an accuracy of ±2 weeks. When maternal history is reliable, Naegele's rule, a simple method of pregnancy dating, remains an accepted standard for calculating gestational age and estimated date of delivery (EDD). The EDD is calculated by counting back three months from the LMP and adding seven days. In addition, EDD is estimated by prenatal sonography if sonography is performed before 22+0 weeks of gestation.

However, accurate menstrual dating and results from sonography performed before 22+0 weeks of gestation may not be available, especially in resource-limited areas. In these cases, gestational age is determined postnatally based on physical examination and neuromuscular assessment. In addition, postnatal assessment is used to confirm the gestational age based on prenatal determination.

PRENATAL ASSESSMENT — The prenatal assessment of gestational age is discussed separately. (See "Prenatal assessment of gestational age, date of delivery, and fetal weight".)

METHODS OF POSTNATAL ASSESSMENT — Gestational age should be assessed soon after delivery. The principal method used to estimate gestational age is the new Ballard score (NBS), which combines physical and neurologic criteria and is easier to administer than the longer Dubowitz method. Examination of the anterior capsule of the lens of the eye and electroencephalography may provide additional information, but are not used routinely.

The physical criteria used are the extent of creases on the sole of the foot, the presence and size of a breast nodule, features of lanugo, characteristics of ear cartilage, and the appearance of genitalia. These criteria can be evaluated immediately after delivery. The neurologic examination described by Amiel-Tison includes the assessment of posture, active and passive tone, and reflexes [1]. An accurate 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.

Dubowitz method — The Dubowitz method was used widely before the development of the NBS. The revised Dubowitz scoring system incorporates 34 physical and neurologic assessments [2,3]. 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. One disadvantage is its overestimation of gestational age in preterm infants. As an example, in a study of 110 preterm infants with a mean gestational age of 28.3 weeks calculated from the last menstrual period (LMP) and best obstetric estimate, the Dubowitz examination overestimated the gestational age by 2.8 weeks [4]. Another problem with this method is the large number of criteria that require evaluation, rendering it difficult to perform on sick or extremely preterm infants and requiring 15 to 20 minutes for completion. The Ballard system has replaced the Dubowitz method as the standard assessment because it is easier to use.

Ballard method — The Ballard system shortened the Dubowitz method to depend upon six physical and six neurologic criteria [5]. The examination is most reliable when it is performed between 30 and 42 hours of age. Similar to the Dubowitz method, the scores of each feature are added to calculate a maturity rating that correlates with gestational age and is accurate within two weeks. This simplified assessment can be accomplished more quickly than can the Dubowitz method (3.5 versus 7 minutes) and is therefore easier to perform on sick infants [5].

However, similar to the Dubowitz method, the Ballard examination may be inaccurate in infants who are preterm, postterm, or small for gestational age. As an example, in a comparison with assessment by early ultrasonography, the Ballard examination resulted in a misclassification of approximately 75 percent of postterm infants because of underestimation of gestational age, and overestimated gestational age in approximately one-half of infants less than 31 weeks [6].

New Ballard score — The Ballard system was modified as the new Ballard score to improve assessment of infants as preterm as 20 weeks (figure 1 and table 1) [7]. This method expands the description of physical and neurologic features and was tested in infants from 20 to 44 weeks gestation. In the initial report, gestational age was slightly greater when estimated by new Ballard score than by LMP and ultrasound (0.15 and 0.32 weeks for the total population and infants less than 26 weeks, respectively). Correlation was similar when the examination was performed up to 96 hours of age in infants of at least 26 weeks gestation, but is best if done prior to 12 hours in infants less than 26 weeks.

However, in a study performed by the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) Neonatal Research Network of 223 infants that were 22 to 28 weeks gestation by accurate menstrual history, the new Ballard score overestimated gestational age by 1.3 to 3.3 weeks [8]. Variation in fetal maturation probably accounts for the inaccuracy of the Ballard examination or other estimates based on physical or neurologic characteristics.

Rapid assessment of gestational age — Over the years, some investigators 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 [9,10]. This technique has also proven beneficial when less experienced health care providers are tasked with estimating gestational age [11]. Clinicians must be aware that neurologic findings in the newly born infant can be affected by certain maternal medications during labor, such as magnesium sulfate or narcotics, which can affect their NBS. Similarly, infants born significantly depressed or those with neuromuscular diseases may have neurologic findings that misrepresent their actual gestational age.

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 [12]. 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 [12,13]. 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 [14-17]. 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 [17]. 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 [18].

EEG and ultrasound or anatomic assessment of gestational age are highly correlated [19-21]. 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) [19]. The maturational progression of EEG background activity occurs at the same rate in the fetus or preterm infant [18].

COMPARISON OF ASSESSMENT TOOLS — It is challenging to identify the most accurate assessment tool for gestational age (GA), as there are no data directly comparing different scoring methods. In addition, the quality of the published literature regarding the accuracy of scoring tools is poor due to a high risk of bias relating to patient selection, testing method, and the reference standard. Nevertheless, the two assessment tools that are the most accurate and commonly used are the Dubowitz and Ballard methods.

In our neonatal intensive care center, we use the new Ballard score to estimate GA, as it is easier to administer and provides more accurate assessment for very preterm infants (less than 28 weeks) than the Dubowitz and original version of the Ballard method. We perform this assessment before 12 hours of age in infants less than 26 weeks gestation, as clinical decisions are influenced by gestational age.

In a systematic review of the literature that identified 18 published assessments used to estimate the GA of newborns, the most commonly used tools were the Dubowitz and Ballard scores, which were validated by either maternal recall of the date of last menstrual period or by dating by prenatal ultrasound [22]. The Dubowitz score dated 95 percent of newborns within 2.6 weeks of ultrasound dating and the Ballard score within 3.8 weeks. Limited data showed that the new Ballard scoring more accurately assessed GA than the original method in infants with GA <30 weeks.

These results suggest that the Dubowitz method is the most accurate tool, primarily due to the positive correlation between the number of parameters assessed and accuracy. However, it is more difficult to administer, requiring training of personnel, and needs more time for completion (usually 15 to 20 minutes).

SUMMARY

●Definition ‒ Gestational age (GA), also called menstrual age or menstrual dating, is the age of the fetus or newborn based upon the completed weeks from the date of the mother's last menstrual period (LMP). (See 'Definitions' above.)

●Prenatal assessment of GA ‒ Prenatal assessment of GA is determined by menstrual history or ultrasound dating. (See 'Overview' above and "Prenatal assessment of gestational age, date of delivery, and fetal weight".)

●Postnatal assessment of GA ‒ Postnatal assessment is based on physical examination and neuromuscular assessment. There are a number of different methods used to determine GA postnatally. (See 'Methods of postnatal assessment' above and 'Comparison of assessment tools' above.)

•The Dubowitz method is the most accurate but is difficult to administer due to its complexity (assessment of 34 parameters) and takes 15 to 20 minutes to complete. It also overestimates GA in very preterm infants (GA <28 weeks).

•The original and New Ballard method shortened the scoring system to 12 parameters (six physical and six neurologic components) (figure 1 and table 1), making it easier to administer over a shorter period of time. The New Ballard method was modified to improve assessment of very preterm infants.

•In our center, we estimate gestational age by postnatal assessment of the newborn by using the new Ballard score. We perform this assessment before 12 hours of age in infants less than 26 weeks gestation since the GA influences clinical decisions. (See 'Comparison of assessment tools' above.)