Methods

This cross-sectional observational study was conducted in the neonatal unit of a tertiary care teaching hospital from March 2015 to May 2016. This unit provides intensive care to sick low birth weight neonates referred from community hospitals of Kolkata and its surrounding states. The Institutional Ethics Committee approved the study.

Five hundred and thirty consecutively selected singleton live-born babies, with a GA of 28 to 41 weeks, were included after obtaining informed written consent from the parents. Neonates for whom reliable GA information was not available were excluded. The following exclusion criteria were applied: mother unaware of the beginning of her last menstrual period (LMP); irregular menstrual cycles before pregnancy; bleeding during the first two months of pregnancy; and difference in GA calculated by LMP and NBS by > 2 weeks. Neonates with congenital anomalies like congenital talipes equinovarus, congenital vertical talus, and other congenital anomalies that hinder anthropometric measurements were excluded along with those with severe perinatal asphyxia. Small for date neonates were also excluded. Also excluded from the study were intrauterine growth restricted (IUGR) and large for gestational age neonates.

Recumbent CHL was recorded to the nearest 1.0 mm using an infantometer. HC was measured between the glabella, anteriorly and along the most prominent point posteriorly by the crossover technique, measured over the parietal eminence. This was performed thrice within 72 hours of birth and the mean value recorded. HC was measured by non-stretchable tape and recorded to the nearest 0.1 cm.

Anterior fontanel area was calculated as anterior fontanel area (in cm2) = ½ × anteroposterior length (cm) × transverse length (cm).

The GA of the study subjects was calculated from the history sheets of their mother, using Naegele’s formula,10 (i.e., addition of 9 months and 7 days to the first day of the LMP) and by NBS11 which was regarded as the gold standard for our study. CHL and HC assessment of GA by NBS was carried out by only one investigator to avoid interobserver bias.

Data were first tabulated in a Microsoft Excel spreadsheet and then analyzed by Statistica version 6 (Tulsa, Oklahoma: StatSoft Inc., 2001) software. Descriptive statistics was generated including mean, standard deviation, and 10th, 25th, 50th, 75th, 90th percentile values. The linear association between GA and anthropometric dimensions was explored using scatter plots and quantified by Pearson’s correlation coefficient (r). Correlation of HC and CHL with GA was strong enough to go for simple and multiple linear regression analysis, and predictive equations generated. The predictive equation from multiple regressions, incorporating both HC and CHL, was applied to a test cohort of 30 children and the margin of error determined from the difference between actual and predicted GA.

Results

Out of 530 neonates, 445 (272 males and 173 females) were recruited for the study as the rest did not meet the inclusion criteria. Figure 1 provides a flowchart of analysis numbers. The demographic summary of the study population is given in Table 1. Further, the sample was divided into term 166 (37 to 41 weeks) and preterm 279 (28 to < 37 weeks). Of the anthropometric parameters measured, namely HC, CHL, and anterior fontanel area, only the former two showed a strong correlation with GA [Tables 2 and 3, and Figures 2 and 3].

HC increased with with increasing GA (i.e., from 24.7±1.4 cm at 28 weeks to 33.8±1.2 cm at 41 weeks). The r-value for correlation of HC with GA was 0.863 and regression equation for calculation of GA from HC was GA (weeks) = 9.2671 + [0.8616 × HC (cm)].

CHL increased steadily with increasing GA from 34.7±1.3 cm at 28 weeks to 49.9±2.5 cm at 41 weeks. The r-value was 0.859 and the regression equation to calculate GA from length was GA (weeks) = 7.2489 + [0.621 × CHL (cm)].

We also derived a multiple linear regression equation to predict gestational age from CHL and HC in a whole cohort. This predictive equation was: GA (weeks) = 4.0244 + [0.4058 × HC (cm)] + [0.4249 × CHL (cm)].

We used this predictive equation to predict the GA from HC and CHL measurements in a separate test cohort of 30 babies. In this cohort, the predicted GA was 33.8±3.5 weeks against the actual GA of 33.4±3.8 weeks. The margin of error was 2.9±1.9%, indicating that the predictive equation was acceptable.

Discussion

Prematurity is a major determinant of neonatal survival. Estimation of GA by methods like recall of LMP is prone to error, and ultrasonic assessment is often difficult to use in resource-poor countries. In developing countries, less than half of neonates undergo any evaluation within 24 hours of birth.12

The NBS score used for GA assessment has both physical and neuronal criteria. It has fallacies as it requires a person trained in pediatrics and furthermore it is a subjective test. Neurological examination requires both skill and training. In contrast, anthropometric measurements collected by health workers have been shown to be more reliable than clinical examination.13,14

A study conducted on a Western Indian population found a strong correlation between GA and HC (r = 0.977).15 Another study observed a strong linear correlation between HC (r = 0.95) and the estimated GA between 25 and 42 weeks.16 In our case, in the age range of Eastern Indian babies between 28 and 41 weeks, the correlation was 0.863.

Another study of neonates delivered in a Western Indian population observed that GA varied from 25 to 42 weeks, with 373 neonates (37.3%) being preterm and 62.7% being term.17 They found a good linear correlation between GA and CHL with r = 0.56. They proposed a linear regression equation: GA (weeks) = 20.06 + [0.34 × CHL (cm)]. We found even stronger correlation (r = 0.859) between CHL and GA and our regression equation is: GA (weeks) = 7.2489 + [0.621 × CHL (cm)].

The strength of our study is that the entire examination was conducted by a medical person trained in examining the newborns and not by any peripheral health worker or traditional birth attendant. Each week of gestational age between 28 and 41 weeks was adequately represented.

This study also has its share of limitations. This sample may not be a true representation of community settings as we conducted the study in a tertiary care center. We had to exclude newborns at 42 weeks gestation due to inadequate numbers. Finally, we did not do a longitudinal follow-up of the babies to determine whether the associations hold at a later age.

Conclusion

HC and CHL measurement are quick and reliable parameters that can be used as an anthropometric surrogate for estimation of GA. These can be used by peripheral health care workers and traditional birth attendants and could be effectively used for identifying and referring high-risk newborns.

Disclosure

The authors declared no conflicts of interest. No funding was received for this study.

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