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INTRODUCTION
The nutritional status and the growth achieved by the newborn until birth have been used as a marker/indicator of early risks of morbidity and mortality1,2, and even of morbidities that may occur at more advanced ages3,4.
Global estimates show that 20 million babies are born with low birth weight in the world every year5 International data also show that around 15 million premature babies are born alive each year6,7. According to the Brazilian National Survey on Labor and Childbirth, the proportion of premature births was 11.3% in Brazil (2011/2012)8, a condition in which it is particularly important to assess the nutritional status and growth of these babies until birth, because, in addition to assessing future risks, it may contribute to understanding possible problems that may have been caused to the fetus during pregnancy9-11, besides to program nutrition more adequate to these babies postnatal needs.
In the postpartum period, it is possible to assess the conditions of the newborn (NB) also based on their gestational age at birth. In this classification, the newborn is categorized as preterm NB (born alive before the 37th week of gestation), early-term NB (born between 37 and 0/7 and 38 and 6/7 weeks of gestation), and full-term NB (born alive between 39 and 41 gestational weeks and 6 days)12. Among preterm newborns, most births correspond to those classified as being late preterm (LPT), whose gestational age is between 34 completed weeks and 36 weeks and 6 days13-15.
The Body Mass Index (BMI), defined as weight in kilograms divided by height in squared meters, is highly correlated with body fat and is a widely used measure for obesity screening at different ages16. However, even though BMI is a good marker of adiposity and is commonly used in older children and adults, there are still gaps in knowledge and there are few studies on the behavior of BMI according to gestational age, particularly in the group currently also considered at risk (late preterm and early-term). Therefore, analyzing BMI in newborns becomes essential, as it is a population with a higher risk for short- and long-term adverse health repercussions.
For these reasons, the objective was to analyze the Body Mass Index (BMI) of late preterm newborns (born at 34 to 36 weeks of gestational age) and of early-term newborns (born at 37 and 38 weeks of gestational age ), as well as the evolution of this index during pregnancy.
METHODS
Study design
This is a descriptive, analytical, and quantitative study based on the database of the project “Biometric characteristics at the birth of babies of young adult women in a municipality with a high human development index”.
Study location and period
Which had data collected between May 2015 and March 2018, at the Maternity Ward of the Taubaté Regional Hospital in the state of São Paulo. The regional hospital is also the UNITAU Hospital School (Taubaté University) and 90% of the births in the city happen there17.
Study population and eligibility criteria
From the 6,321 infants born alive during this period, newborns from single gestations, with low risk, and no pathologies were selected.
All babies born alive who were included for analysis were born from single gestations between 34 and 38 gestational weeks, from women with no complications during pregnancy, and presented at least the following data in their medical records: date of birth, mother’s age, or mother’s birthdate, type of gestation, type of labor, gestational age (GA), sex, birth weight, birth height.
Newborns who did not fit the described characteristics and/or did not present any record of the data established in the inclusion criteria were excluded from the study. Additionally, those with body measurements with more or less than 3 standard deviations from the average of the studied population were excluded.
At the end of these assessments, of the 6,321 newborns, 2,468 newborns remained (39.0% of the total) in the study, which included all 528 late preterm newborns (PTT NB) and all 1,940 early-term newborns (ET NB). Among the newborns included in the study, 1,173 (47.5%) were female, and 1,295 (52.5%) were male.
Data collection
Anthropometric measurements were taken by healthcare professionals trained in their execution, measurement, and recording. Values were collected from birth registration books and, when necessary, complemented by checking information from the hospital’s electronic medical records.
The measurement of the weight and height of newborns was carried out following the Technical Norms of the Food and Nutritional Surveillance System (SISVAN)18. For weight in kilograms, the scale was first turned on before the infant was placed on the equipment. After reaching zero on the scale, the newborn was carefully positioned in its center without a diaper, and the weight value was patiently observed until it stabilized on display for accurate registration. The height was measured in centimeters (cm) using an anthropometric ruler, with the newborn lying on a firm surface. With the help of a parent, the newborn’s head was firmly supported against the fixed part of the equipment, with the neck and chin apart, shoulders, buttocks, and heels in contact with the surface, knees extended, and the soles of the feet in contact with the surface of the moving part of the equipment18,19.
Data analysis
All necessary data were duly filled in from the newborns’ medical records and copied to an Excel® spreadsheet for later consistency analysis. Comparisons of measures of central tendency and dispersion of values were performed, in addition to correlations and regressions (Mann-Whitney Test; D’Agostino - Pearson Test; Spearman Correlation Coefficient). For the statistical analysis, the following software programs were used: MedCalc v. 20.218 - 64 bit, CurveExpert Pro v. 2.0.4, GraphPad Prism v. 6.0, and GraphPad Instat v. 3.1.0.
All tables and graphs in this study had their titles simplified, as they refer to the population of mothers and newborn babies with gestational age (GA) between 34 and 38 weeks, from the city of Taubaté, between 2015 and 2018. We used Olsen IE20 as a reference to analyze the BMI measures, weight, and height of newborns in table 2, table 3 and table 4.
Table 2 : Distribution of BMI z-score of late preterm NB (LPT NB) and early-term NB (ET NB)
| BMI | LPT NB | ET NB | Reference |
|---|---|---|---|
| (z-score) | Values | ||
| Median | 0.65 | 0.58 | 0.00 |
| CI 95% | 0.57 to 0.74 | 0.54 to 0.64 | - |
| Average | 0.62 | 0.58 | - |
| (DP) | (0.9) | (0.8) | - |
| p10 | -0.60 | -0.44 | -1.28 |
| p25 | 0.02 | 0.06 | -0.68 |
| p75 | 1.27 | 1.15 | 0.68 |
| p90 | 1.93 | 1.62 | 1.28 |
| N | 528 | 1940 | - |
CI* Confidence Interval / Comparison of medians Mann-Whitney Test p=0.1439 (non-significant).
Table 3 : Birth weight Z-score distribution, in late preterm NB (LPT NB) and early-term NB (ET NB)
| Weight | LPT NB | ET NB | Reference |
|---|---|---|---|
| at birth | (z-score) | (z-score) | Values* |
| Median | -0.01 | -0.08 | 0.00 |
| CI 95% | -0.11 to -0.08 | -0.12 to -0.03 | - |
| Average | 0.05 | -0.09 | - |
| (DP) | (0.9) | (0.8) | - |
| p10 | -1.09 | -1.11 | -1.28 |
| p25 | -0.65 | -0.62 | -0.68 |
| p75 | 0.66 | 0.43 | 0.68 |
| p90 | 1.42 | 0.96 | 1.28 |
| N | 528 | 1940 | - |
Comparison of medians: Mann-Whitney Test p=0.0192 (significant)
Table 4 : Distribution of height Z-score at birth in late preterm NB (LPT NB) and early-term NB (ET NB)
| Height | LPT NB | ET NB | Reference |
|---|---|---|---|
| At birth | (z-score) | (z-score) | Values* |
| Median | -0.55 | -0.71 | 0.00 |
| CI 95% | -0.67 to -0.49 | -0.75 to -0.69 | - |
| Average | -0.5 | -0.72 | - |
| (DP) | (0.8) | (0.7) | - |
| p10 | --1.68 | -1.62 | -1.28 |
| p25 | -1.16 | -1.19 | -0.68 |
| p75 | -0.01 | -0.26 | 0.68 |
| p90 | 0.59 | 0.21 | 1.28 |
| N | 528 | 1940 | - |
Comparison of medians: Mann-Whitney Test p=0.0001 (significant)
Ethical and legal aspects of the research
The study was sent to Plataforma Brasil, in accordance with Resolution No. 466, of December 12, 201221, and authorized under opinion number 5,427,258.
RESULTS
The distribution of BMI between male and female late preterm newborns is similar. It is also similar among early preterm newborns of both sexes (table 1). In the same table, it is observed that in terms of absolute BMI values, late preterm newborns have a lower median BMI (12.6 kg/m2) than the BMI of early-term newborns (13.6 kg/m2).
Table 1 : BMI distribution in kg/m2 of late preterm NB (LPT NB) and early-term NB (ET NB) according to sex
| BMI | LPT NB | LPT NB | LPT NB | ET NB | ET NB | ET NB |
|---|---|---|---|---|---|---|
| (kg/m2) | Male | Female | All | Male | Female | All |
| Median | 12.7 | 12.6 | 12.6 | 13.7 | 13.5 | 13,6 |
| CI* 95% | 12.4 to 13.0 | 12.3 to 12.7 | 12.4 to 12.7 | 13.6 to 13.8 | 13.4 to 13.6 | 13.6 to 13.7 |
| Average | 12.7 | 12.5 | 12.6 | 13.7 | 13.5 | 13.7 |
| (DP) | (1.5) | (1.5) | (1.5) | (1.3) | (1.3) | (1.3) |
| p10 | 10.9 | 10.5 | 10.8 | 12.2 | 12.1 | 12.1 |
| p25 | 11.7 | 11.5 | 11.6 | 12.9 | 12.8 | 12.8 |
| p75 | 13.7 | 13.4 | 13.5 | 14.6 | 14.5 | 12.8 |
| p90 | 14.9 | 14.3 | 14.5 | 15.4 | 15.3 | 15.4 |
| N | 298 | 230 | 528 | 997 | 943 | 1940 |
CI*: Confidence Interval.
In table 2, considering the distribution of all values, there is no difference in the z-score between the late preterm and early preterm groups. As for the distribution in percentiles, the values are systematically superior to the reference values.
The distributions of birth weight in the two groups are different (table 3). Considering their gestational age, late preterm newborns are proportionally heavier than early-term newborns, with a median practically equivalent to the reference value.
When comparing heights in z-score, the late-preterm newborns median is bigger than the early-term newborns median (table 4). Still, both values are smaller than the reference value median, which shows a greater distance from the height median than the one observed for the weight.
This behavior can be observed in the graphs of figure 1, figures A, B, C, and D, in which changes in the evolution trend of both parameters can also be observed from 37 weeks of gestational age.
Figure 1 : BMI evolution tendency, weight, and height at birth for LPT NB and ET NB, with gestational age evolution
Regarding the BMI in absolute values, kg/m2, (figure A), there is a tendency for a proportionally higher growth rate in late preterm newborns followed by a deceleration in growth in early-term newborns as gestational age increases.
In figure B, late preterm newborns have higher values of BMI z-score in relation to the reference values, indicating that those with lower gestational ages tend to be proportionally a little higher than their peers (reference values) and that after the 37th gestational week, there is a reduction in the BMI growth speed.
Regarding weight (figure C), it is noted that from the 37th week of gestational age, the weight tends to decrease with the reference value, while the height growth (figure D) tends to remain stable.
DISCUSSION
In this study, late preterm newborns were proportionally larger than early-term newborns. Although there is a growing evolution in absolute BMI values for both genders, which is expected considering the gestational age, median values above those proposed by the reference value21 are a consequence of a birth height below the reference value median at all gestational ages.
When analyzed in z-score, the BMI shows similar median values between preterm and early-term newborns, but when considering the reference value, the values of these two groups are lower than those observed in full-term newborns, regardless of their sex.
These findings corroborate the study that evaluated the evolution of fetal weight carried out in Campinas—SP. In this study, Cecatti et al.22 found that the newborn’s weight gain between the 27th and 38th gestational week was on average 200g/week and that there was a reduction in the speed of weight gain from the 39th to the 42nd week of pregnancy, which also affects the BMI values at birth.
This BMI behavior suggests that during pregnancy there is a non-proportional growth between fetus weight and height, which favors weight gain during the second pregnancy half. According to Cecatti et al.22, maternal physiological characteristics, such as parity, weight, and height, as well as maternal pathologies can influence fetal growth, especially in the 3rd trimester of gestation.
An adequate growth assessment is necessary to indicate how satisfactorily the intrauterine needs of babies have been met20,23. In this sense, it is essential to be attentive to preterm newborns with low birth weight or when they are too small for their gestational age because they are the ones who have the highest risk of negative impacts, including for their neurodevelopment20-26.
The evolution analysis of BMI percentiles and/or z-score in the study of sample populations, according to the gestational age, is important to analyze whether the 50 percentile (median) and the 10 and 90 percentiles, usually used to assess the intrauterine growth of newborns, show a trend like the one observed for the newborn group as a whole20,27,28.
A relative limitation of this study is the selection of a specific urban community from the State of São Paulo population, which makes it difficult to extrapolate the data to the general population. Another limitation that is common to many studies involving growth studies is the accuracy of the measured data since the data were not collected strictly with a focus on the research. On the other hand, it is important to consider that they were collected by a team of professionals trained to do so correctly, as a routine of their service.
Furthermore, a relevant positive aspect of the research is that it resorted to a study population (universe), not a sample, which results in adequate representativeness of the results, even if it is exclusively for the newborn population in Taubaté. Although this may represent an obstacle to a broader generalization of the values obtained, it is no different from the difficulties in generalizing the results that are also observed when resorting to convenience samples, which also do not fully reflect the population universe that one wishes to analyze.
CONCLUSION
The distribution of absolute BMI values (kg/m2) shows an increase with the evolution of gestational age for both male and female newborns, with BMI values for boys, generally, higher than those for girls.
In the late preterm and early-term newborns, considered at higher risk, when compared to full-term newborns, the BMI increases more intensely in the late preterm period (34th to 36th week of pregnancy), tending to slow down its growth speed in the period that corresponds to the early-term, 37th and 38th weeks of gestation.
