Birth weight and ponderal index percentiles for twins based on sex and chorionicity in a center of Guangdong Province, China

Introduction

Birth weight is a critical indicator of an infant’s health and can have significant implications for morbidity later in life. First, it is associated with a range of adverse health outcomes across the lifespan. These outcomes include increased risks of infant mortality, and developmental delays or neurodevelopmental disorders, all-cause adult mortality, necrotizing enterocolitis (1), cardiovascular disease, stroke, and type 2 diabetes (2-4). The overdiagnosis of intrauterine growth restriction (IUGR) can lead to several harms, including but not limited to the following: unnecessary medical interventions, psychological and financial burden, waste of medical resources.

Second, corrected birth weight percentiles, rather than absolute weights, are associated with detrimental effects. A birth weight percentile chart is used by obstetricians and neonatologists to evaluate whether the birth weight of neonates is appropriate for gestational age or not. The assessment of weight in conjunction with height provides a more comprehensive reflection of nutritional status, as reflected by body mass index (BMI) (5) and ponderal index (PI). PI has been used for assessing fetal growth and is regarded as a better measure of complicated reflecting factors than the birth weight percentile. It has been reported that neonates with a low PI have higher morbidity and mortality (6). Research has demonstrated that the PI has a moderate correlation with neonatal body fat percentage, serving as a crucial parameter for assessing adipose tissue accumulation in newborns. When integrated with birth weight percentiles, the PI can more precisely mirror fetal growth patterns and nutritional status (7).

Twin births constitute 2–4% of all births worldwide (8,9) and the incidence of twin pregnancies is on the rise due to increased use of assisted reproductive technology. The adverse outcomes of twin pregnancies are significantly greater than those of singleton pregnancies particularly in terms of low birth weight (10). It is important to assess whether the birth weight and PI of twins are appropriate for their gestational age or not. Previous studies have suggested that the use of twin-specific growth charts is likely a better approach for preventing the overdiagnosis of IUGR in pregnancies with twins (8,11).

Birth weight percentiles for twins, stratified by sex, have been published and updated in various countries and regions, including Argentina (12), Australia (13), Canada (14), Germany (15), Japan (16), Norway (17), Taiwan (8), Wuhan city of China (9) and northern China (18). However, the data analysis did not consider the chorionicity of twins, which has been considered as a decisive factor in the outcomes of twin pregnancies.

The primary objective of this study is to construct and present current sex-and chorionicity-specific birth weight and PI percentile charts for twins, utilizing data from a neonatal population in Guangzhou, a major city in southern China. This study could serve as a valuable tool for assessing the growth patterns of twins in southern China. We present this article in accordance with the STROBE reporting checklist (available at https://tp.amegroups.com/article/view/10.21037/tp-24-308/rc).

Methods

Study population

We conducted a retrospective analysis of sex-specific live-born twins birth weight and PI based on different chorionicities at Guangdong Women and Children Hospital between 2015 to 2020. Each record included information regarding the mode of delivery, neonatal sex, gestational age at birth, chorionicity, birth weight, and length. All the data were collected from the patients’ clinical records.

We included the live-born twins whose gestational ages ranged from 25 to 40 weeks. Gestational age was estimated via a combination of the last-menstrual-period (LMP) and ultrasound scans conducted during the first trimester. Chorionicity was assessed through ultrasound scans before to 13⁺⁶ weeks of gestational age, and this assessment was supplemented by pathological examination of the placenta to determine the number of placentas or layers of amniotic and chorionic membranes. The birth weight of newborns was meticulously measured within one hour of birth by a professional midwife via an electronic scale, with the results accurately recorded in the medical records down to the nearest gram. The PI was calculated for all study subjects via the following formula: birth weight (g) ×100/[birth length (cm)]³ (6).

Ethical approval and informed consent

The study was performed in compliance with the approved guidelines and regulations. Ethical approval for the research was granted by the Research Ethics Committee of Guangdong Women and Children Hospital, Guangzhou, China (No. 201701048). The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013), and informed consent was obtained from all participating pregnant women.

Statistical analysis

Statistical analysis was conducted via R v.4.0.0 (R Development Comprehensive R Archive Network 2020). We calculated the 3, 10, 50, 90 and 97 percentiles for neonatal birth weight and the PI between 25 and 40 weeks of gestational age, stratified by sex and chorionicity. In cases where critical data were missing and the precise information cannot be retrieved from the original records, the entire record was excluded from the analysis. One previous study has demonstrated that the generalized linear model is appropriate for presenting birth weight percentile curves, and thus, this model was employed in this study (18). The derived percentile curves were smoothed via the Lambda Mu Sigma (LMS) method, implemented through the generalized additive models for location, scale and shape (GAMLSS). The LMS method initially estimated the three parameters of the Box-Cox power exponential distribution of the measurement. Based on the percentile values for each gestational week calculated via R software, we applied GraphPad Prism 10.1.2 (GraphPad Software, USA) to plot the curves for twin birth weights and the PI.

We compared the neonatal parameters of twins, stratified by sex and chorionicity, at the same gestational age via a t-test, following the assessment of normality. All the statistical tests were conducted at a significance level of 0.05.

Results

A total 3,433 live-born twins were included in this study, comprising 366 monochorionic girls, 405 monochorionic boys, 1,237 dichorionic girls and 1,425 dichorionic boys. The medical records of still birthed and miscarried of twin fetuses were excluded from the study. We calculated the mean gestational age and birth weight of the neonates, and PI were calculated according to neonatal sex, chorionicity, mode of delivery and the presence of preterm infants, as summarized in Table 1.

Our findings indicated that the mean birth weight of monochorionic twins was significantly lower than that of dichorionic twins (t=7.57, P=0.006), and that the gestational age at delivery for monochorionic twins was significantly lower than that for sex-matched dichorionic twins (t=18.71, P<0.001). Among female twins, the mean gestational age at delivery for monochorionic twins was significantly earlier than that of dichorionic twins (t=6.099, P=0.014). The mean birth weight of monochorionic female twins was lower than that of their dichorionic counterparts, although the difference was not significant (t=2.704, P=0.100). In contrast, for male twins, the mean birth weight of monochorionic twins was lower than that of dichorionic twins (t=5.528, P=0.019), and the gestational week of delivery was earlier for monochorionic twins (t=13.591, P<0.001).

The average and the percentiles (3rd, 5th,10th, 50th, 90th, 95th and 97th) for estimated birth weight and PI of both monochorionic and dichorionic twin girls and boys at various gestational ages are presented in Tables 2-5. Additionally, the smoothed reference curves for neonatal birth weights and PI of monochorionic and dichorionic twin girls and boys are depicted in Figures 1-8.

Figure 1 Percentile chart of birth weight based on gestational age for monochorionic female twins.

Figure 2 Percentile chart of ponderal index based on gestational age for monochorionic female twins.

Figure 3 Percentile chart of birth weight based on gestational age for monochorionic male twins.

Figure 4 Percentile chart of ponderal index based on gestational age for monochorionic male twins.

Figure 5 Percentile chart of birth weight based on gestational age for dichorionic female twins.

Figure 6 Percentile chart of ponderal index based on gestational age for dichorionic female twins.

Figure 7 Percentile chart of birth weight based on gestational age for dichorionic male twins.

Figure 8 Percentile chart of ponderal index based on gestational age for dichorionic male twins.

The results revealed notable differences between singletons and twins based on chorionicity. Specifically, the 50th percentile birth weight for dichorionic male twins was consistently greater than that for female twins across all gestational ages. Conversely, the 50th percentile birth weight for monochorionic male twins was lower than that of female twins prior to 31 weeks of gestation.

We also compared the median values of twin birth weights at various gestational ages with those of singletons, using data from a nationwide singleton cross-sectional study in China (19) as a reference (see Tables 6,7). The results revealed that the median birth weights of singleton boys before 27 weeks were lower than those of both monochorionic and dichorionic twins. However, after 27 weeks of gestational age, the median birth weights of singletons were significantly exceeded those of both monochorionic and dichorionic twins of the same sex at equivalent gestational ages.

Discussion

This retrospective study analyzed the birth weights of neonatal twins, stratified by sex and chorionicity, from 25 to 40 weeks of gestational age in Guangzhou city, China, over the period from 2015 to 2019. This study provides a gestational-age-specific weight percentile spectrum for twins, differentiated by chorionicity. The findings offer valuable insights for assessing the growth and development progress of twin-born newborns within a specific geographic area.

First, birth weight serves as a crucial indicator of fetal intrauterine development (20). Pediatricians and neonatologists commonly utilize neonatal birth weight curves to assess an infant’s growth status and to provide medical or health care advice, based on whether the neonatal development falls within the normal range (21). Research has demonstrated that an inappropriate PI is linked to adverse neonatal outcomes, with a PI ranging from the 10th to the 90th percentile being considered appropriate (6). Furthermore, obstetricians believe that birth weight percentile charts specially designed for twins merit particular attention (9,21,22). Accurate neonatal twin birth weight percentile charts, tailored for different chorionicities and specific gestational weeks are instrumental in evaluating and extrauterine health risks, particularly for monochorionic twins. It has been reported that assessing twins via singleton birth weight percentiles rather than twin-specific birth weight percentiles may lead to increased neonatal unit admission and ventilation support. Conversely, when twin-specific standards are applied, there is an observed increase in neonatal mortality rates. Therefore, the appropriate selection of birth weight standards can help avoid unnecessary hospitalizations and mechanical ventilation for neonates. This highlights the importance of using accurate birth weight standards to optimize neonatal care and outcomes (23).

Second, while some may argue that the birth weight percentile curves based on different chorionicities are not necessary, the evidence suggests that their benefits are indeed significant. A study has shown that the incidence of stillbirth and neonatal death in smaller dichorionic twins classified as small for gestational (SGA) is significantly greater than that in those who are SGA (24). Additionally, monochorionic pregnancies, as opposed to dichorionic pregnancies, carry a greater risk of adverse outcomes, including stillbirth, neonatal death morbidities, and adverse neurodevelopmental sequelae (25). Therefore, distinguishing between SGA twins, who may require intensive care, and non-SGA twins, who can receive routine newborn care, is essential. Birth weight percentile curves that account for chorionicity and gestational age are thus invaluable tools for perinatal health care professionals and researchers in identifying high-risk fetuses and neonates in need of special care and close monitoring (8). These findings indicate that by using sex-and chorionicity-based percentile charts for twin birth weight and the PI, fewer newborn twins will require intensive care unit (ICU) admission, enabling more precise identification of those who need additional medical attention. Consequently, this approach can alleviate the mental and financial stress on parents.

Third, birth weight for gestational age is a critical indicator of the adequacy of fetal growth. Restricted fetal growth is likely part of the causal pathway to all forms of attention-deficit/hyperactivity disorder (ADHD) (26). In this study, we observed that the birth weight of monochorionic twins was lower than that of dichorionic twins, and that most twins weighed less than singleton neonates at the same gestational age. Previous studies reported that infants who undergo multiple pregnancies are generally smaller and more likely to be premature than singleton infants are (9,27,28). Many authors emphasize the necessity for a specific assessment for neonates born form multiple pregnancies, highlighting the need for a reliable range of population attributable birth weight percentile values (29-32).

Finally, our study reaffirmed the fundamental principle that birth weight chats for neonates exhibit steeper slopes with increasing gestational age during the third trimester, a pattern observed in both singleton and twin pregnancies in previous research (8,33,34). Additionally, we discovered that the calculated 50th percentile birth weight values of twin boys were greater than those of girls, with the same level of chorionicity, with the exception of monochorionic twins before 31 weeks of gestational age. Extensive previous research has shown that boys are generally born heavier than girls in singleton pregnancies (35,36) and in most twin pregnancies (37). Furthermore, we found that the mean birth weight of dichorionic twins was greater than that of monochorionic twins of the same sex. This observation is consistent with findings from previous studies conducted on different populations (22,38). Conversely, we noted that the calculated 50th percentile values for monochorionic twin boys were lower than those for girls before 31 weeks of gestational age. The reason for this discrepancy warrants further investigation.

Several limitations of the present study warrant mentioning. First, this was a single-center study, which may limit its generalizability to local regions. Second, the sample size was relatively small for monochorionic twin infants, a constraint influenced by the chorionicity ratios observed in twin pregnancies. We acknowledge that our study’s focus on the raw data of birth weight and the PI in twins has certain limitations. By not including variables such as maternal BMI, weight gain during pregnancy, educational attainment, and economic status, our analysis does not capture the full spectrum of factors that may influence birth weight and the PI. As a result, while our data accurately represent the distribution of these metrics in a twin birth population, they fall short of elucidating the underlying causes and trends that could be associated with variations in these indices. This limitation underscores the need for future research to incorporate a broader range of maternal and socioeconomic factors to provide a more comprehensive understanding of the determinants of birth weight and PI in twins.

Following the groundwork laid by this research, future studies should be conducted in multiple centers and across diverse populations to better understand the broader implications of our findings.

Conclusions

Neonatal birth weight percentile charts that consider both sex and chorionicity offer a valuable tool for assessing the well-being of twin infants. The utilization of twin growth charts based on chorionicity can help prevent the over-diagnosis of SGA in twins who are otherwise growing normally.

Acknowledgments

The authors express their gratitude to the staff members of the Department of Obstetrics and Medical Genetics Centre at Guangdong Women and Children Hospital, as well as the parents and their children who generously participated in the study.

Funding: This study was supported by Guangzhou Science and Technology Program Project (No. 202201011768).

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://tp.amegroups.com/article/view/10.21037/tp-24-308/rc

Data Sharing Statement: Available at https://tp.amegroups.com/article/view/10.21037/tp-24-308/dss

Peer Review File: Available at https://tp.amegroups.com/article/view/10.21037/tp-24-308/prf

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://tp.amegroups.com/article/view/10.21037/tp-24-308/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was performed in compliance with the approved guidelines and regulations. Ethical approval for the research was granted by the Research Ethics Committee of Guangdong Women and Children Hospital, Guangzhou, China (No. 201701048). The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013), and informed consent was obtained from all participating pregnant women.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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