Why is it important for the nurse to know the infants gestational age and how it compares with birth weight?

Journal Article

William M. Callaghan,

*Correspondence to Dr. William M. Callaghan, Division of Reproductive Health, Centers for Disease Control and Prevention, 4770 Buford Highway, MS K-23, Atlanta, GA 30341 (e-mail: ).

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Received:

24 September 2009

Accepted:

29 December 2009

Published:

25 February 2010

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  William M. Callaghan, Patricia M. Dietz, Differences in Birth Weight for Gestational Age Distributions According to the Measures Used to Assign Gestational Age, American Journal of Epidemiology, Volume 171, Issue 7, 1 April 2010, Pages 826–836, https://doi.org/10.1093/aje/kwp468

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Abstract

Measures of infant size for gestational age are used to determine the immediate health-care needs of the newborn and increasingly to predict long-term health concerns. Size for gestational age at birth reflects energy exposure and transfer and placental function during pregnancy; being born small for gestational age or large for gestational age has been associated with adverse neonatal and infant outcomes and developmental outcomes during childhood and beyond (1–8). Although the obstetrician may be most concerned about the growth of a fetus compared with all other fetuses, the pediatrician may care more about the size of the newborn compared with all other newborns at a similar gestational age. Consequently, there has been extensive discussion about basing the appropriateness of size for gestational age on fetal growth curves instead of gestational age-specific birth weights for preterm infants (9, 10). The central issue of these discussions is that infants delivered prematurely are likely to be growth restricted and, hence, their birth weight distributions differ from distributions of fetuses undelivered at the same gestational age; fetuses, on average, weigh more than infants of the same age. However, at the population level, the obstetric information used to assess interval fetal growth and indicators of excess or restricted growth is not available. Hence, population-based birth weight for gestational age standards is routinely used to monitor national or regional trends in large-for-gestational-age and small-for-gestational-age births and to monitor progress toward improved infant health (11).

Population-based standards for infant size at specific gestational ages depend on accurate assessments of birth weight and gestational age. The accuracy of the latter measure, based primarily on the last menstrual period (LMP) as recorded on birth certificates, has been questioned (12–15). Public-use natality files have 2 measures of gestational age, including LMP and the clinical estimate of gestational age from states using the 1989 version of the birth certificate, and LMP and the obstetric estimate of gestational age from states using the 2003 version of the birth certificate. Instructions for the clinical estimate require that gestational age be estimated by the birth attendant and that it not be computed from the LMP (16). The more specific instructions for the obstetric estimate require that birth attendants base the estimate on all perinatal factors including ultrasound and specifically exclude the neonatal examination as a determinant of the estimate (17). Although previous studies have documented discrepancies between LMP- and early ultrasound-based estimates of gestational age (13, 18), evaluations of clinical estimate and obstetric estimate have not been performed, nor have comparisons of fetal weight for gestational age and birth weight for gestational age based on different measures.

The purpose of this analysis was to compare birth weight distributions and measures of small for gestational age and large for gestational age among the different approaches with measurement of gestational age. Comparison of these methods will highlight inherent differences in these approaches and inform future studies that focus on small for gestational age and large for gestational age when these measures are used as exposures or outcomes.

MATERIALS AND METHODS

We used the 2005 public-use US natality file from the Centers for Disease Control and Prevention's National Center for Health Statistics (NCHS) for this analysis. The natality file contains information from birth certificates received by NCHS from all states and the District of Columbia. States have discretion concerning which data elements they collect, but there are 2 basic templates for birth registration. The first is the 1989 revision of the US standard birth certificate, and the second is the more recent 2003 revision. It is pertinent to this analysis that the 1989 revision contains the “clinical estimate” field for gestational age and the 2003 revision contains the “obstetric estimate” field. In 2005, 12 states reported births using the 2003 revision; these states represented 31% of all births. Further details of the 2005 birth cohort (19) and the 2003 birth certificate revision are available elsewhere (20). The analysis was limited to singleton births with known birth weights, no missing information for LMP-based, obstetric, or clinical estimates of gestational age, and estimated gestational ages between 20 and 44 weeks inclusive for the estimate of interest.

We compared 4 measures of gestational age: 1) the NCHS standard measure of gestational age based mostly on LMP, 2) the clinical estimate available on the 1989 revised birth certificate (37 states, New York City, and the District of Columbia; California used the 1989 revision but did not report the clinical estimate resulting in exclusion of California births), 3) the obstetric estimate available on the 2003 revised birth certificate (12 states; New York does not include New York City), and 4) a “gold standard” described below (Appendix Table 1). With a few exceptions, the standard measure of gestational age in national statistics is based on the recorded LMP except when the LMP date is missing or when birth weight is extremely implausible for the LMP estimate, that is, when the birth weight is very high and the gestational age is very short or the birth weight is very low and the gestational age is very long. For these exceptions, the clinical or obstetric estimate of gestational age may be used as the standard measure of gestational age. Gestational age is imputed when there is a valid month and year for LMP, but the day of the month is missing. Details of the gestational age edits are published elsewhere (21). For 2005, the LMP-based standard estimate of gestational age was based on clinical or obstetric estimate for only 5.8% of births, the vast majority because the LMP date was missing (19). We refer to this LMP-based estimate hereafter as the standard estimate.

We created a subcohort of births to use as a gold standard population for gestational age measurement. In this subcohort, we imposed the requirement that the standard estimate and the clinical or obstetric estimate of gestational age must agree within 1 week of one another regardless of the direction of agreement and that there was no record of a congenital anomaly on the birth certificate. Because early prenatal care is likely associated with better recall of LMP and access to early ultrasound, we further imposed the requirement that the woman initiated prenatal care by the third month of pregnancy. When these conditions were met, we used the standard estimate and designated this as the gold standard estimate. Because there is controversy about the appropriateness of birth weight instead of fetal weight as the basis for understanding size for gestational age, we also compared the median birth weight for gestational age using each measure of gestational age with estimates of median fetal weight for gestational age based on a commonly used fetal weight model (22). We determined birth weight distributions for each gestational age between 20 weeks and 44 weeks (inclusive) for each of the 4 measures of gestational age. Comparisons among measures were stratified by states using the 1989 birth certificate revision and by states using the 2003 birth certificate revision in an effort to maintain consistent samples for comparison purposes. We also compared the demographic characteristics of mothers with births included in the standard, clinical, obstetric, and gold standard estimates for states that used the 1989 birth certificate revision and for states that used the 2003 birth certificate revision. We used plots to compare the gestational age-specific median and the 10th and 90th percentile birth weights with one another for each measure of gestational age. Absolute and proportionate differences between birth weights for gold standard estimates of gestational age and for clinical, obstetric, and standard estimates were calculated. In an effort to determine if differences in gestational age measures are affected by characteristics of women, we compared median birth weight for gestational age distributions for each revision according to strata of race/ethnicity and age. We explored the effect of substituting clinical or obstetric estimates for standard gestational age estimates by restricting the analysis to those records where the standard estimate was determined solely from the LMP. The effect of imputing gestational age was explored by restricting the analysis to those records where there was no imputation. Finally, to assess how trends in gestational age measurement might affect estimates of size for gestational age, we compared median birth weights in 2005 with median birth weights in 1990, a time in the early implementation of the 1989 revision, for different measures of gestational age.

RESULTS

In 2005, there were 3,998,533 singleton births, of which 2,769,548 were births in states using the 1989 birth certificate revision. Among these, 1,453 were missing birth weight, 25,659 were missing standard estimates of gestational age, and 537,004 were missing clinical estimates of gestational age (most of which were from California, which did not collect information on the clinical estimate); 2,231,354 births had complete information. There were 1,228,985 births in states using the 2003 birth certificate revision. Among these, 1,900 were missing birth weight, 2,857 were missing standard estimates of gestational age, and 6,092 were missing obstetric estimates of gestational age; 1,221,317 births had complete information. Finally, 577 births were not included because both the standard estimate and the clinical or obstetric estimate of gestational age was less than 20 weeks or greater than 44 weeks. A total of 3,452,094 births were ultimately included; 2,231,071 were from the 1989 revision and 1,221,023 were from the 2003 revision.

The demographic characteristics of the infants’ mothers differed from one another according to which birth certificate revision was used and depending on if information on the birth certificate permitted classification as a gold standard estimate (Table 1). Women identified by the 2003 revision were more likely to be Hispanic. Women included in the gold standard estimate were slightly older and more educated and were more likely to be married. By definition, women included in the gold standard estimate had early initiation of prenatal care.

Table 1.

Characteristics, by Percentage, of US Women With a Singleton Birth in 2005 and Women Who Were Included in the Gold Standard Gestational Age Category by Birth Certificate Revisiona

a

Records with known birth weight, nonmissing standard and clinical or obstetric estimate of gestational age, and where at least 1 gestational age estimate was between 20 and 44 weeks.

b

States that used the 2003 revision include Florida, Idaho, Kansas, Kentucky, Nebraska, New Hampshire, New York (excluding New York City), Pennsylvania, South Carolina, Tennessee, Texas, and Washington.

c

Other race/ethnicity includes American Indian/Alaskan Native, Asian/Pacific Islander, and black women and white women of unknown Hispanic ethnicity.

d

Education is ascertained according to years on the 1989 revision and by degree on the 2003 revision. For this analysis, 4 years of high school was considered a degree, and 4 years of college was considered a degree.

e

This variable includes 10 months for the 2003 revision.

Table 1.

Characteristics, by Percentage, of US Women With a Singleton Birth in 2005 and Women Who Were Included in the Gold Standard Gestational Age Category by Birth Certificate Revisiona

a

Records with known birth weight, nonmissing standard and clinical or obstetric estimate of gestational age, and where at least 1 gestational age estimate was between 20 and 44 weeks.

b

States that used the 2003 revision include Florida, Idaho, Kansas, Kentucky, Nebraska, New Hampshire, New York (excluding New York City), Pennsylvania, South Carolina, Tennessee, Texas, and Washington.

c

Other race/ethnicity includes American Indian/Alaskan Native, Asian/Pacific Islander, and black women and white women of unknown Hispanic ethnicity.

d

Education is ascertained according to years on the 1989 revision and by degree on the 2003 revision. For this analysis, 4 years of high school was considered a degree, and 4 years of college was considered a degree.

e

This variable includes 10 months for the 2003 revision.

For both the 1989 and 2003 birth certificate revisions, the median birth weights for gestational age were virtually identical for the gold standard and clinical and obstetric estimates, respectively (Figure 1). In contrast, the median weight for gestational age for the standard estimate exceeded that of the gold standard between the gestational ages of 28 and 36 weeks, and it was lower than the gold standard beyond 40 weeks’ gestation. The median birth weight differences between the gold standard and the standard estimates were similar for both revisions, ranging from 3.4% to 17.0% for the 1989 revision and from 3.0% to 21.2% for the 2003 revision at 36 and 30 weeks’ gestation, respectively. The same differences for other measures ranged from 0.0% to 2.5% (Table 2). Results were similar for race/ethnicity-specific (non-Hispanic black, non-Hispanic white, Hispanic) and age-specific (<20, 20–29, 30–39 years) plots of median birth weights. Using the standard estimates resulted in consistently greater birth weights for preterm births than the clinical and obstetric and clinical estimates in the preterm range (data not shown). Restricting the analysis to records where clinical or obstetric estimates were not substituted for LMP estimates resulted in a further increase in birth weight for gestational age for the standard estimate compared with birth weights where substitution occurred (data not shown). There was no effect when the analysis was restricted to records that did not have imputation of LMP.

Table 2.

Absolutea and Percentb Differences Between US Birth Weights by Gold Standard Estimate of Gestational Age and Clinical/Obstetric, Obstetric, Clinical, and Standard Estimates of Gestational Age Between 28 and 36 Weeks of Gestation, 1989 and 2003

a

Absolute difference in grams.

b

Percent difference in parentheses.

Table 2.

Absolutea and Percentb Differences Between US Birth Weights by Gold Standard Estimate of Gestational Age and Clinical/Obstetric, Obstetric, Clinical, and Standard Estimates of Gestational Age Between 28 and 36 Weeks of Gestation, 1989 and 2003

a

Absolute difference in grams.

b

Percent difference in parentheses.

Figure 1.

Median birth weights for gestational age by 3 measures, 1989 birth certificate revision (A) and 2003 birth certificate revision (B), United States, 2005.

The differences between the standard estimate and all other measures were far less dramatic for infants in the 10th percentile for birth weight, with the exception of infants born at 32–34 weeks’ gestation whose standard estimate birth weights differed from the gold standard by 3.1%–7.1% (Figure 2; Table 2). When the standard estimate was used, the 90th percentile birth weights at all gestational ages less than 38 weeks were markedly greater than the 90th percentile birth weights for all other measures of gestational age (Figure 3). Birth weight differences between the standard estimates and the gold standard were as much as 130% while, except for the 30-week obstetric estimate, birth weights for obstetric and clinical estimates differed by less than 10% from gold standard estimates (Table 2). Moreover, the plot of birth weight for gestational age deviated from the anticipated sigmoid shape for the standard measure of gestational age. For all other measures, the curve was sigmoid, with the exception of a deviation at 30 weeks’ gestation that was most prominent for the obstetric estimate (Figure 3). In general, birth weights were lower for standard estimates beyond 40 weeks, particularly at 41 and 42 weeks, compared with clinical and obstetric estimates.

Figure 2.

Tenth percentile birth weights for gestational age by 3 measures, 1989 birth certificate revision (A) and 2003 birth certificate revision (B), United States, 2005.

Figure 3.

Ninetieth percentile birth weights for gestational age by 3 measures, 1989 birth certificate revision (A) and 2003 birth certificate revision (B), United States, 2005.

Median birth weights for standard and gold standard estimates of gestational age were compared with estimated fetal weights for gestational age (Figure 4). For preterm infants between 28 and 33 weeks by the gold standard estimate, the median birth weights at each gestational age were less than those of comparably aged fetuses. The opposite was true when the standard estimate of gestational age was used; median birth weights were equal to or heavier than estimated fetal weights.

Figure 4.

Median birth weight for gestational age using the standard estimate and the gold standard estimate for the 1989 and 2003 birth certificate revisions, United States, 2005, and the median estimated fetal weight model for gestational age.

Median birth weights for gestational age using all measures were compared for 1990 and 2005 (Figure 5) and were virtually indistinguishable for all measures except the standard estimate. For both years, newborns dated with the standard estimates had median birth weights for gestational age that were notably higher than all other gestational age measurements. By using the standard estimate of gestational age, birth weights for 1990 were generally higher than birth weights for 2005.

Figure 5.

Median birth weights for gestational age by standard, clinical, and obstetric estimates, United States, 1990 and 2005.

DISCUSSION

Our analysis of singleton births in 2005 suggests that using the current composite measure of clinical and obstetric estimates of gestational age in US natality files results in birth weight distributions that are virtually identical to a gold standard for estimated gestational age. Moreover, we found negligible differences between the birth weight distributions when the clinical estimate was compared with the obstetric estimate, despite the fact that the instructions for obtaining these estimates differ and that the demographic characteristics of the populations differed. Last, size-for-gestational-age distributions that result from the historical construct for the clinical estimate appear no different from those resulting from the current constructs for clinical estimate or obstetric estimate. However, using the standard measure of gestational age based on LMP resulted in size-for-gestational-age distributions that were substantially different from the gold standard. To the extent that agreement between 2 sources of estimated gestational age is a reasonable basis for a gold standard, these results suggest that clinical or obstetric estimates may be preferable for establishing population-based size-for-gestational-age norms.

The differences between birth weights for the standard estimate of gestational age and all other measures were most notable at the high end of the birth weight distribution. This finding is consistent with the previously described bimodal distribution of birth weights among preterm births, found in official national statistics where gestational age is based primarily on LMP (14, 23). This bimodal phenomenon is characterized by a dominant distribution with a mode consistent with the expected birth weight for gestational age and a smaller secondary distribution with a mode consistent with the expected birth weight for term infants. Thus, it is not surprising that the standard measure based primarily on LMP did not compare well with the gold standard, and that the direction of the difference was such that birth weights among preterm births were higher, on average, when the standard estimate was used, compared with clinical or obstetric estimates. However, the magnitude of the difference was remarkable; weights for gestational age at the 90th percentile by standard estimates were as much as twice the weights for gestational age by gold standard estimates. Similarly, the lower birth weights among postterm infants defined by standard estimates compared with obstetric and clinical estimates are consistent with more misclassification at the extremes of gestational age when using standard estimates (12).

Median birth weights from standard estimates of gestational age were also higher among preterm infants than the estimated fetal weights, while using the gold standard estimate resulted in preterm infant birth weights lower than the estimated fetal weights. The former observation is inconsistent with the well-documented clinical observation that preterm newborns, on average, weigh less than fetuses at the same gestational age (24–27), while the latter observation is consistent with this body of literature. Moreover, although birth weight differences at the 10th percentile were more similar among all of the gestational age measures, birth weights for the standard estimate were consistently higher among moderately preterm newborns. Hence, using the standard estimate of gestational age underestimates the true difference between the preterm newborn weight distribution and the preterm fetal weight distribution. Although the estimated fetal weights we referenced came from a widely used source based on a specific population (22), the discrepancy between estimated fetal weights and birth weights for preterm infants has been noted using fetal weight estimates from other sources (24, 25).

A strength of this analysis is that it involves almost all births in the United States, and thus it is applicable to discussions of how size-for-gestational-age norms ought to be established in the United States. However, California does not have a field on the birth certificate that reports clinical estimate or obstetric estimate in 2005. Hence, no births from California were included in this analysis, resulting in exclusion of 1 in 8 births in the United States. Although the population of California likely has some unique characteristics, it seems unlikely that size for gestational age at birth would differ so dramatically that it would have affected the outcome of this analysis. The demographic characteristics of births used for clinical, obstetric, and gold standard gestational estimates differed. However, in spite of these differences, clinical, obstetric, and gold standard size-for-gestational-age estimates are virtually indistinguishable from one another; regardless of race/ethnicity or age, the patterns of agreement between clinical/obstetric estimate and the gold standard estimate were similar, and the standard estimate consistently differed, regardless of birth certificate revision. Therefore, the different demographics of births to women in California would unlikely change any of this study's findings.

All large data sets have the potential for misclassification of data elements, and there is surprisingly little information about how data elements such as gestational age estimates are gathered at the facility level. We also assumed that states that adopted the 2003 birth certificate revision included the element of obstetric estimate of gestational age, an assumption that has not yet been validated. Moreover, recording information is subject to mistakes. For example, a recent population-based analysis of births conceived through artificial reproductive technology reported birth weight distributions using the reported date of conception as the measurement of gestational age. Although apparent misclassification was markedly attenuated—as evidenced by the marked dimunition of the second mode for birth weight among preterm births—there were still a few infants with apparent term birth weights among the preterm births (15). The authors postulated that data entry errors might explain that observation. In our analysis, we found a similar phenomenon, where 90th percentile birth weights for clinical and obstetric estimates at 30 weeks’ gestation were higher than the gold standard. In further analyses (data not shown) using the gold standard estimate as a reference, we found that 6% of births at 30 weeks by obstetric estimates had birth weights that exceeded the 10th percentile for birth weight at 40 weeks. At 29 and 31 weeks of gestation, only 3% of newborns had weights exceeding the 10th percentile for 39 and 41 weeks, respectively. This suggests the substitution of the digit 3 for the digit 4, resulting in the misclassification of a 40-week infant as 30 weeks’ gestation. A similar “bump” at the 90th percentile was observed by Kramer et al. (28) in an assessment of birth weight for gestational age in Canada, where the gestational age source was believed to be largely based on obstetric ultrasound. To the extent that data entry errors might account for this observation, data entry checks regarding birth weight and gestational age might be useful adjuncts for improving the quality of information on birth certificates. However, it remains unclear why this effect occurs so prominently for the obstetric estimate.

Birth weight for gestational age is a fundamental perinatal measure at the clinical and population levels. For the clinician, comparing the birth weight of the newborn with the birth weight of its peers informs neonatal care (9). At the population level, understanding the risk factors for being born small or large for gestational age, documenting trends, and defining population norms for birth weight have been traditional public health functions. For example, understanding the relation between population-based guidance for weight gain in pregnancy and its relation to fetal growth requires population-based norms for size for gestational age. To these ends, population norms ought to be based on the most accurate data available for birth weight and gestational age.

A previous concern with the clinical estimate was the potential that it could be based on neonatal examination, which is an imprecise approach to measuring gestational age. However, the percentage of clinical estimates based on the neonatal examination is unknown. This study found that the distributions of birth weight for gestational age using the obstetric estimate (for which instructions explicitly stated not to use the neonatal examination) and using the clinical estimate were very similar. The internal consistency of these 2 estimates and their agreement with the gold standard suggest that the current composite measure using clinical and obstetric estimates is a satisfactory measure of gestational age for the purpose of establishing standards and compares well with the historic clinical estimate of gestational age. Although there continues to be discussion about the best measurement of gestational age for establishing population-based, gestational age-specific birth rates, using the measurement of gestational age that depends almost solely on the recorded first day of the mother's LMP for establishing population norms for gestational age-specific birth weights may need to be reconsidered.

Abbreviations

Abbreviations

 

• LMP

 

• NCHS

  National Center for Health Statistics

Author affiliation: Division of Reproductive Health, Centers for Disease Control and Prevention, Atlanta, Georgia (William M. Callaghan, Patricia M. Dietz).

A portion of this report was presented at the 15th Annual Maternal and Child Health Epidemiology Conference, Tampa, Florida, on December 9, 2009.

The findings and conclusions in this report are those of the authors and do not necessarily represent the official position of the Centers for Disease Control and Prevention.

Conflict of interest: none declared.

Appendix Table 1.

Definitions of Measures for Estimating Gestational Age

Appendix Table 1.

Definitions of Measures for Estimating Gestational Age

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American Journal of Epidemiology Published by Oxford University Press on behalf of the Johns Hopkins Bloomberg School of Public Health 2010.

American Journal of Epidemiology Published by Oxford University Press on behalf of the Johns Hopkins Bloomberg School of Public Health 2010.

Topic:

• birth certificates
• birth weight
• gestational age
• infant
• obstetrics
• birth
• gold standard

Why is it important for the nurse to know the infants gestational age?

How does gestational age affect birth weight?

Would birth weight be a better indicator than gestational age?

When should the gestational age of the infant be determined?