The impact of glycosylated hemoglobin and 75-g oral glucose tolerance test glucose levels on birthweight percentile

SUMMARY OBJECTIVE: Our research objective was to validate and contribute further evidence to the studies regarding large for gestational age and birthweight percentile by examining oral glucose tolerance test and glycosylated hemoglobin levels in both healthy women and those with gestational diabetes mellitus. METHODS: This retrospective cohort study was conducted at a tertiary care hospital involving 106 women who delivered at gestational week 36 or later between February 2022 and February 2023. Maternal, obstetric, and neonatal data were collected from the participant's medical records. Large for gestational age and non-large for gestational age groups were compared. Correlation analysis was used to determine associations among oral glucose tolerance test, glycosylated hemoglobin levels, and the birthweight percentile. RESULTS: Mothers of neonates in the large for gestational age category had higher body mass indexes before pregnancy (p=0.002) and delivery (p=0.003), as well as a higher incidence of gestational diabetes mellitus (p=0.027). Mothers of male large for gestational age infants had higher fasting plasma glucose and glycosylated hemoglobin levels compared to male non-large for gestational age infants (p=0.007 and p=0.004, respectively). There was a weak positive correlation between fasting plasma glucose levels and birthweight percentile in the overall group (r=0.342, p<0.006). Further analysis by gender showed a weak positive correlation between birthweight percentile and fasting plasma glucose and glycosylated hemoglobin values in male newborns (r=0.393, p=0.004 and r=0.373, p=0.006, respectively). CONCLUSION: Our study has established a clear association between the birthweight percentile in male infants and the levels of glycosylated hemoglobin and fasting plasma glucose measured during oral glucose tolerance test. It is imperative to devise potential strategies aimed at achieving optimal glycosylated hemoglobin and fasting plasma glucose parameters to effectively reduce the frequency of large for gestational age in male infants.


INTRODUCTION
Birthweight is a crucial factor in predicting the survival of newborns and infants, as well as an important indicator of pregnancy outcomes 1 .Additionally, it has been observed that birthweight is associated with the future risk of cardio-metabolic diseases in the offspring.It was reported that 10.8% of newborns are large for gestational age (LGA), exhibiting a high birthweight percentile 2 .
Maternal glucose level is an important factor in prenatal health, as it plays a vital role in providing essential nutrients to both pregnant women and their fetuses 3 .GDM is a common metabolic disorder that refers to any degree of glucose intolerance occurring during pregnancy 4 .This condition has been linked to the passage of glucose through the placenta to the fetus, leading to increased fetal insulin production and subsequent excessive growth, resulting in LGA infants 5 .
Although the diagnostic "gold-standard" for GDM is the oral glucose tolerance test (OGTT), glycosylated hemoglobin (HbA1c) is a commonly used measure to evaluate long-term glucose regulation 6 .Studies have established an association among GDM, high glucose levels, third-trimester HbA1c values, and LGA infants 7 .
While a significant portion of the research on prenatal glucose regulation has primarily concentrated on diabetic pregnancies, it is important to note that negative consequences have also been observed in pregnant individuals without diabetes 8 .However, there is a scarcity of studies, particularly focusing on pregnant women who do not have GDM.Our research objective is to validate and contribute further evidence to the studies regarding LGA and birthweight percentile by examining OGTT values and HbA1c levels in both healthy women and those with GDM.

Statistical analysis
All statistical analyses were performed using IBM SPSS version 20.0 for Windows (IBM Corp., Armonk, NY, United States).Shapiro-Wilk and Kolmogorov-Smirnov tests were used to assess the normality assumption.Continuous variables were presented with mean±standard deviation or median (IQR: interquartile range).Categorical variables were summarized as counts and percentages.Independent-samples t-test or Mann-Whitney U test was used for comparison between groups.The association between two categorical variables was examined by the chisquare test.Spearman correlation analysis was used to determine the associations between continuous variables.A p-value <0.05 was considered statistically significant.

RESULTS
This study included a total of 106 women.The mean maternal age was 30 ±5.6 years.The gestational age at the time of delivery varied between 36 and 40 weeks, with a mean of 38 weeks.The mean birthweight was 3400±458 g.There were 53 male and 53 female newborns.Maternal, obstetric, and neonatal characteristics of the study group are displayed in Table 1.Among 106 women, 21.6% (n=23) were diagnosed with GDM.
LGA was present in 33% of the study group (n=35) while 67% (n=71) had non-LGA births.Table 2 displays the comparison of LGA and non-LGA groups for maternal, obstetric, and neonatal variables.There was a statistically significant difference in various factors such as pre-pregnancy BMI (p=0.002),BMI before delivery (p=0.003),GDM (p=0.027), and polyhydramnios (p=0.003).Additionally, the FPG values were significantly higher in the LGA group (p<0.001).LGA was more prevalent among male infants (p=0.013).One woman in the LGA group and four women in the non-LGA group used insulin (p=1).Only one woman in the LGA group and two women in the non-LGA group had unregulated glucose levels at the time of birth.
Upon categorizing the groups by gender, it was observed that mothers of male LGA infants had higher levels of FPG and HbA1c compared to male non-LGA infants (p=0.007 and p=0.004, respectively) (Table 3).
According to correlation analysis, no significant correlations were found between birthweight percentile and the results of 1-h OGTT, 2-h OGTT, and HbA1c (p=0.195,p=0.546, and p=0.245 respectively), but FPG level showed

DISCUSSION
In our research, mothers of LGA infants tended to have a higher pre-pregnancy BMI and elevated FPG values.Our study found a correlation between the birthweight percentile and the neonatal gender, indicating a higher likelihood of classifying male newborns as LGA.Furthermore, we observed a positive correlation between HbA1c and FPG values and the birthweight percentile, particularly in male infants.In our study, both maternal pre-pregnancy BMI and BMI before delivery were higher in the LGA group.Several research findings have indicated that a high pre-pregnancy BMI is the primary indicator for the delivery of a baby with fetal overgrowth.According to the study by Chen et al., the risk of macrosomia was significantly higher in women who were overweight or obese before pregnancy compared to women with a normal weight 11 .In another study, obese women were found to have a 2.27-fold higher likelihood of developing LGA compared to women who were not obese 12 .This observation was not limited to women with GDM but also included women with normal glucose tolerance.A large study from China underlined that male neonates, overweight, and obesity were linked with an increased risk of delivery higher than 4,000 g in nondiabetic women 13 .
The presence of GDM can have significant implications for both the mother and the fetus.The IADPSG criteria were formulated based on the findings of the hyperglycemia and adverse pregnancy outcomes (HAPO) study.The HAPO study revealed a consistent and progressive correlation between fasting and post-load maternal glucose levels and the occurrence of LGA infants, high adiposity, and elevated concentrations of cord-blood C-peptide 14 .
During pregnancy, blood glucose levels of 70 mg/dL or lower in the OGTT have been associated with lower birth weight, smaller head circumference, and shorter body length in infants compared to those born to mothers with normal blood glucose levels 15 .Individuals with a lower glucose challenge test (GCT) result during the second trimester had a higher incidence of SGA newborns compared to those with a normal result 16 .On the other hand, studies have underlined that the infants born to women with high FPG levels during OGTT exhibited a significantly greater average birthweight and birthweight percentile when compared with nonGDM women and a higher risk of LGA 17,18 .Zhao et al. found that as FPG levels increased by 1 mmol/L, there was a corresponding rise in the birth weight percentile 19 .They also observed a 0.70 times decrease in the risk of SGA, while LGA increased by 1.80 times.Glucose measurements exhibited a linear correlation with LGA, with the strongest link observed for FPG.In line with the aforementioned studies, we found that median FPG levels were higher in mothers of LGA fetuses.Further analysis showed that FPG levels were correlated with the birthweight percentile in our study.On the contrary, in a recent study, there was no significant correlation observed between FPG, plasma glucose 2 h after the OGTT, postprandial blood glucose values at 28 weeks of gestation, and birthweight 20 .According to Yang et al., fetal growth is mainly influenced by postload glucose levels rather than FPG values 21 .Nevertheless, the methodologies employed in the latter two studies differ from our own.
In a recent study, no significant relationship was found between the levels of maternal HbA1c before 20 weeks of gestation and the birthweight of the neonate in women without pre-existing diabetes 22 .Silke et al. observed a positive correlation between maternal HbA1c levels and neonatal birthweight 23 .However, this association was found to be significant only in pregnancies with male fetuses.Similarly, our research revealed a positive correlation between maternal HbA1c, FPG, and birthweight percentile, but exclusively among male neonates.It is important to note that birthweight percentile rather than birthweight was evaluated in our study.A prior investigation presented proof that the glucose metabolism of mothers could potentially be influenced by the gender of the fetus, even in pregnant women with normal glucose tolerance 24 .FPG was found to be higher in mothers of male infants.The presence of a male fetus was linked to diminished β-cell function, which refers to the reduced ability of pancreatic cells involved in counteracting insulin resistance 25 .The study's strengths lie in the balanced representation of both genders, which enhances the statistical validity.Notably, our study encompassed a substantial number of healthy women, which has offered additional understanding and the benefit of placing various pathological glucose response patterns into context.
Acknowledging the study's limitations, reliance on self-reported pre-pregnancy height and weight may introduce recall bias.To minimize this, the participant's pre-pregnancy weights and heights were recorded at the onset of pregnancy.The study population consists primarily of individuals of Caucasian ethnicity, limiting the generalizability of our findings to other ethnic groups.The variable "birthweight" is influenced by numerous predictors and confounders, but our research is limited by a restricted set of predictor variables.Data on patient's dietary habits, physical activity, parental birthweight, and paternal body habitus are lacking, which could potentially impact the outcomes of the study.

CONCLUSION
Our study has established a clear association between the birthweight percentile in male infants and the levels of HbA1c and FPG measured during OGTT.Pregnant women with abnormal glucose tolerance carrying a male fetus may require closer monitoring of fetal growth.It is imperative to devise potential strategies aimed at achieving optimal HbA1c and glucose parameters to effectively reduce the frequency of LGA in male infants, which should be considered in future research.

ETHICAL APPROVAL
This study was carried out at Kocaeli University Hospital.All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards.This study received approval from the Ethics Committee of Kocaeli University (KU GOKAEK 2023/17.16).Informed written consent was obtained from the patients.

Table 1 .
Maternal, obstetric, and neonatal characteristics of the study population.

Table 2 .
Characteristics of the study population according to birthweight percentile categories.The impact of oral glucose tolerance test and glycosylated hemoglobin levels on birthweight percentile Data are shown as mean ± SD or median (25th-75th quartile) or number and percentage (%).*p<0.05indicates statistical significance.a Independent-samples Mann-Whitney U test.b Independent samples t-test.c Chi square test.LGA: large for gestational age; BMI: body mass index; GDM: gestational diabetes mellitus; FPG: fasting plasma glucose; OGTT: oral glucose tolerance test; GA: gestational age.

Table 3 .
Comparison of glycosylated hemoglobin and oral glucose tolerance test values in large for gestational age and non-large for gestational age groups based on neonatal gender.are shown as mean ± SD or median (25th-75th quartile).*p<0.05indicates statistical significance.a Independent-samples t-test.b Mann-Whitney U test.LGA: large for gestational age; FPG: fasting plasma glucose; OGTT: oral glucose tolerance test.