“Low birth weight” was first defined in 1977 by the World Health Organization (WHO) as “<2500 g”.1 Since then, this terminology has mostly been abandoned in favor of “small for gestational age (SGA)” representing birth weight (BW) <10th percentile for gestational age. At the other end of the growth spectrum, “large for gestational age (LGA)” represents newborns with BW >90th percentile for gestational age.2,3 Today, SGA and LGA designations have served as tools to measure risk of morbidity in term infants, which may affect immediate management after birth. For example, infants deemed SGA will have serial screening for hypoglycemia and other conditions (e.g., cytomegalovirus infection), as it is assumed that this weight cutoff will differentiate high- from low-risk newborns. However, the validity of these cutoffs has been questioned.4,5

To further complicate matters, various growth curves were developed using different methodologies and populations, such as cross-sectional vs. longitudinal data, international vs. United States, and preterm vs. term newborns.6,7,8,9,10,11,12,13 Growth curves differ especially at the extreme lower and upper percentiles, where major decisions are made.14 The delineation of SGA/LGA, therefore, varies based on the specific growth curve used.

The electronic medical record is another factor that confuses growth designations. Many hospitals’ electronic medical record systems default to only one type of growth curve rather than having an array of growth curves based on gestational age and often lag in updating them (e.g., Fenton 2003 vs. 2013). This further hinders appropriate clinical practice and uniformity among units and institutions.

Recently at our own institution, the newborn nursery (NBN) began using the Fenton preterm growth curve6 for term infants for the purpose of categorizing size for dates at birth, while the level IV neonatal intensive care unit (NICU) continued to use the WHO growth curve.12 This growth curve switch resulted in different term newborns being screened for hypoglycemia based upon which unit they were admitted to at the same institution.

While newborns delivered at 39 weeks gestation have similar 10th and 90th percentile weights on the WHO and Fenton growth curves, as gestational age decreases closer to early term, such as at 37 weeks gestation, the Fenton growth curve cutoffs for SGA and LGA status are 300–400 g lower than the WHO growth curve values regardless of sex (Fig. 1). A similar but opposite gap is observed between the WHO and Fenton growth curves as gestational age  increases. Therefore, when using the Fenton vs. the WHO growth curve, fewer newborns delivered at 37 weeks gestation are deemed SGA, whereas at 41 weeks gestation, more infants are labeled as SGA.

Fig. 1: Comparison of weight-to-gestational age cutoffs (grams) for 10th and 90th percentile of WHO, Fenton, and Olsen growth curves.
figure 1

The left and right graphs show comparisons for the 10th and 90th percentile weight cutoffs for males and females, respectively, for different growth curves.

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Since term newborns at our institution have growth parameters identified differently based upon the care unit of admission, we sought to evaluate current practices at other U.S. University hospitals with the hypothesis that there is significant variation in the use of growth curves both within (NICU vs. NBN) and between hospitals.

To evaluate the usage of growth curves in NICUs and NBNs across the nation, we surveyed NICU Chiefs and Medical Directors in University institutions with a Neonatal-Perinatal Medicine fellowship program via e-mail (using Neonatology Chief and NICU Medical Director list serves) from September to December 2019. Those who did not respond to the initial survey received up to two e-mail reminders. Chiefs and NICU Medical Directors were asked to identify which growth curves were used in their NICUs and NBNs, for preterm infants (<37 weeks gestation) and term newborns, and at birth to determine size for gestational age and for longitudinal growth. Each question was accompanied by the following growth curves as choices: INTERGROWTH-21st (2015),9 Fenton Preterm Growth Chart (2013),6 Olsen (2010),13 WHO (2006),12 Other (specify), and <2500 g (SGA) and >4000 g (LGA). For the questions about NBNs, additional answer choices included “no preterm infants in NBN” and “do not have a NBN” (Supplementary Appendix S1 (online)).

Based on our survey, the WHO (2006),12 Fenton Preterm Growth Chart (2013),6 and Olsen (2010)13 growth curves were the most commonly used growth curves in NICUs and NBNs around the country. To demonstrate differences in the 10th and 90th percentile BW cutoffs, data from these growth curves were either obtained from the original publication (Olsen 2010)13 or web-based electronic supplements (WHO).15 For the Fenton 2013 growth curve cutoffs, the Actual Age Calculator v8, championed by the original author on the University of Calgary website,6,16 was used to determine the closest weight in grams, which corresponded to 10th percentile (z-score of −1.3) or 90th percentile (z-score of +1.3). The Actual Age Calculator v8 uses the actual age of the infant at the time the anthropometric data was obtained, specific to the day (i.e., 37 weeks and 1 days) rather than averaging data into completed weeks.16 This calculator is most appropriate for clinical applications, where gestational weeks and days are available, and most accurately reflects the goals of our research.

One hundred institutions with a Neonatal-Perinatal Medicine fellowship program were contacted through the e-mail list serves. Eighty-one institutions responded to the survey. Of those, 79 responded to the questions regarding their NBNs. Five institutions did not have NBNs and three institutions did not have preterm infants in their NBNs. For preterm infants, 100% of NICUs use a preterm growth curve (93% use the Fenton growth curve) to categorize size for dates at birth, compared to 86% of NBNs (77% use the Fenton growth curve, Fig. 2).

Fig. 2
figure 2

Comparison of growth curves used in NICUs and newborn nurseries for preterm and term infants.

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However, for term infants at birth, there was a divide between which growth curve was used in NICUs and NBNs. Fifty-one percent (41/81) of NICUs and 50% (37/74) of NBNs use a term growth curve (i.e., the WHO growth curve) for term infants, followed by 41% (33/81) and 35% (26/74) of NICUs and NBNs, respectively, use the Fenton preterm growth curve (Fig. 2). Intra-institutional agreement between NICUs and NBNs was only 76% (i.e., 24% of NICUs and their associated NBNs used different growth curves at birth).

No surveyed NICUs use the criteria of “<2500 g and >4000 g” for SGA/LGA categorization. For NBNs, only two institutions use the specific BW cutoffs for term infants.

There was no difference between the growth curves used for size-for-dates purpose vs. for measuring longitudinal growth for both preterm and term newborns (data not shown).

This survey of U.S. institutions with Neonatal-Perinatal Medicine fellowship programs found that most preterm newborns have birth parameters plotted on preterm growth curves (most used the Fenton growth curve) irrespective of care unit admitted to. In contrast, for term infants, only half of the surveyed hospitals use term growth curves (mostly the WHO growth curve) in the NICU. Despite that most admissions to NBNs are term infants, still only half of term newborns were plotted on term growth curves, similar to NICUs. This inconsistency in using preterm growth curves for half of term newborns is less of a problem at 50 weeks gestation when the 2013 Fenton growth curve essentially matches the WHO growth curve.6 On the other hand, for infants born at 37 and 41 weeks gestation, there are very large BW discrepancies identifying SGA/LGA status between the Fenton growth curve, plotted based on gestational age, and the WHO growth curve, which does not distinguish between gestational age at birth (Fig. 1). For example, a male newborn at 38 weeks gestation with a BW of 3800 g would be considered LGA if plotted on the Fenton growth curve, but not if plotted on WHO growth curve. Moreover, if this infant was to be transferred inter-institutionally from NICU to NICU, there is only a 50% chance that the LGA diagnosis will remain.

Based on our survey, there is no consensus on the appropriate growth curve to use for term infants in the U.S. The variability in the type of growth curve used at birth for term newborns and the inherent difference between the Fenton growth curve, which adjusts for gestational age, in comparison to the WHO growth curve, which does not, can significantly alter whether a newborn is considered small or large for gestational age. Strong historical support for the use of the 2500 and 4000 g cutoffs for the definition of low birth weight and LGA, respectively,17,18 and their continued use in subsequent studies19 suggest that using these BW cutoffs may be a convenient surrogate for defining SGA and LGA. For the purpose of stratifying postnatal morbidity and mortality, we call upon the AAP Committee on Fetus and Newborn and other governing bodies to opine on which growth curve to use for term infants at birth. While a consensus is pending regarding abnormal growth status in term infants, perhaps eliminating growth curves as a confounder and use BW cutoffs of <2500 g and >4000 g, respectively, to determine SGA and LGA status may be in order.18,20