INTRODUCTION

Post-term births are defined as those occurring at 42 weeks of gestation or later. The rate of post-term birth varies markedly across countries. For instance, among singleton births in Europe, this frequency ranged from 0.4% to 8.1% [1]. In Japan, the post-term birth rate for singleton births in 2022 was reported to be 0.1% [2]. The method used to calculate gestational age can influence the reported post-term birth rate, potentially contributing to the differences observed among countries [1].

Post-term delivery is significantly associated with increased risks of perinatal and maternal complications [3]. Conditions such as asphyxia and intellectual disability occur more frequently among those born post-term compared to term births [4]. Furthermore, post-term birth has been positively linked with thinness, attention deficit, and hyperactivity in early childhood [5,6]. While some studies have indicated a significantly positive association between post-term birth and neonatal mortality [7,8], others have not found such a significantly positive association [9-11]. The association has not been explored in Japanese populations. Consequently, additional research is warranted to investigate this association within Japan.

The causes of post-term birth remain incompletely understood, although genetic factors and socio-demographic characteristics appear to be associated [12]. Research has explored the maternal and infant characteristics linked to post-term birth [11,13-16], with identified factors varying by country. In Japan, limited studies have examined these characteristics. One study targeting primiparous mothers found that females aged 35 years or older were more likely to experience postterm birth than those aged 25-29 years [17]. Another study indicated a higher rate of post-term birth among non-Japanese mothers compared to Japanese mothers [18]. However, these studies relied on data from a single institution. Thus, the characteristics associated with post-term birth in Japan should be investigated using nationwide data.

The present study aimed to evaluate the association between post-term birth and neonatal mortality, as well as to identify factors associated with post-term birth in Japan, using a nationwide dataset.

METHODS

Data

This analysis utilized birth and mortality data from the Vital Statistics of Japan, spanning the years 2017 to 2022. The dataset included all live births and deaths reported to the Japanese government. The Ministry of Health, Labour and Welfare provided this information in accordance with Article 33 of the Statistics Act of Japan. Specifically, information on gestational week, birth weight, sex, prefecture, municipality, date of birth, and multiplicity for infants was used from the birth data. Additionally, information on age, date of birth, marital status, nationality, and parity for mothers were also used from the birth data. Moreover, information on infant mortality status, date of birth, age at death, birth weight, multiplicity, prefecture, municipality, sex, and the mother’s date of birth was used from the mortality data.

Post-term birth was defined as that occurring at 42 weeks of gestation or later, and neonatal mortality referred to death within the first 28 days of life. This analysis utilized data on neonatal mortality attributed to illness, as detailed information was available only for those neonatal deaths. Maternal age was categorized into the following groups: under 20, 20-24, 25-29, 30-34, 35-39, and 40 years or older. Maternal nationality was classified as either Japanese or non-Japanese. Parity was divided into 2 categories: primiparous and multiparous. The occupation of the highest earner in the household was recorded as the household occupation. The categories for household occupations included farmer; self-employed; full-time worker at a company with fewer than 100 employees; full-time worker at a company with 100 or more employees, public servant, or corporate board member; other type of worker; and unemployed.

Statistical Analysis

Data were linked between birth records and neonatal mortality data using shared information to append neonatal mortality status to the birth records. The linking variables included prefecture, municipality, infant sex, infant birth date, and maternal birth date. In the linkage, one-to-one matched pairs were identified as infants with neonatal mortality. In cases where multiple birth records corresponded to a single neonatal mortality record, birth weight was also taken into account in the linkage.

The numbers of births occurring at term (37-41 gestational weeks) and after term were tallied by birth characteristics. Subsequently, the post-term birth rate was computed by these characteristics. Additionally, the number and rate of neonatal deaths were calculated for both post-term and term births. To explore the relationship between post-term birth and neonatal mortality, a log-binomial regression model was employed. This model included the following explanatory variables: postterm birth status, infant sex, maternal age group, parity, maternal nationality, maternal marital status, household occupation, and birth year. Adjustments were made for infant sex, maternal age group, parity, maternal nationality, maternal marital status, and household occupation, because these variables have been demonstrated to relate to infant mortality in Japan [19]. An adjustment was also made for the birth year since post-term birth and neonatal mortality rates have been decreasing over time in Japan. Infant birth weight was not adjusted, as it is considered an intermediate variable between gestational age and neonatal mortality rather than a confounder. A log-binomial regression model was also applied to examine the association between birth characteristics and post-term birth, using infant sex, maternal age group, parity, maternal nationality, maternal marital status, household occupation, and birth year as explanatory variables. These variables represent maternal and infant characteristics available in the Vital Statistics data. Adjusted risk ratios (aRRs), 95% confidence intervals (CIs), and p-values were calculated, with a p-value of less than 0.05 considered to indicate statistical significance.

We analyzed the birth data of infants born between January 2017 and November 2022 to calculate the risk of neonatal mortality. Specifically, we included singleton infants born at 37 weeks of gestation or later. We excluded data with missing values, as the rate of missing data was low. All statistical analyses were performed using R version 4.1.3 (R Foundation for Statistical Computing, Vienna, Austria). The statistical results were generated by the authors using data provided by the Japanese Ministry of Health, Labour and Welfare. These are not statistics published by the Ministry.

Ethics Statement

This study received approval from the Institutional Review Board for Clinical Research at Kyushu University (No. 22221-08). Informed consent was deemed unnecessary for this research as it utilized official statistical data provided by the Ministry of Health, Labour and Welfare, in compliance with the Statistics Act of Japan.

RESULTS

Figure 1 presents the flowchart of study population selection. Two infants displayed identical characteristics and were matched with a single neonatal mortality record. However, it was unclear whether these infants experienced a neonatal mortality event in the analysis, so they were excluded. Ultimately, the analysis included a total of 4 698 905 live births.

Table 1 presents the number (percentage) of births and the rate of post-term births according to various characteristics. The overall rate of post-term birth was 0.15%. Among maternal age groups, those under 20 years old exhibited the highest rate of post-term birth. Unmarried females experienced a postterm birth rate more than twice that of married females. Additionally, the rate of post-term births among primiparous females was over 4 times higher than among multiparous females. Regarding household occupations, unemployed households and those including a farmer reported the highest and second-highest post-term birth rates, respectively. Female infants had a higher post-term birth rate than male infants.

Supplementary Material 1 illustrates the results of linking birth records with neonatal mortality data. A total of 3936 cases of neonatal mortality were matched with corresponding birth records.

Table 2 presents the number and rate of neonatal deaths, along with the results of the regression analysis exploring the relationship between post-term birth and neonatal mortality. The neonatal mortality rates for post-term and term births were 2.5 and 0.3 per 1000 births, respectively, indicating a higher rate of neonatal mortality for post-term births. The aRR of post-term birth for neonatal mortality was 8.07 (95% CI, 5.06 to 12.86), demonstrating a significantly positive association.

Table 3 presents the results of regression analysis exploring the relationship between birth characteristics and post-term birth. Female infants were found to experience a higher risk of post-term birth compared to male infants (aRR, 1.21; 95% CI, 1.15 to 1.27). Female aged 35-39 and those 40 years or older also faced a greater risk of post-term birth than female 30- 34 years old, with aRRs of 1.21 (95% CI, 1.14 to 1.29) and 1.20 (95% CI, 1.09 to 1.33), respectively. In contrast, the risk was lower for female under 20, those aged 20-24, and those aged 25-29, with aRRs of 0.75 (95% CI, 0.61 to 0.93), 0.82 (95% CI, 0.76 to 0.90), and 0.82 (95% CI, 0.77 to 0.87), respectively, when compared to the 30-34-year-old group. Primiparous females were found to have a higher risk of post-term birth than multiparous females, with an aRR of 4.77 (95% CI, 4.50 to 5.06). Additionally, non-Japanese mothers had a higher risk compared to Japanese mothers, with an aRR of 1.20 (95% CI, 1.06 to 1.36). Unmarried mothers also faced a higher risk than married mothers (aRR, 1.82; 95% CI, 1.61 to 2.07). Regarding occupation, households with farmers, full-time workers at smaller companies with fewer than 100 employees, other types of workers, and unemployed individuals were at a higher risk of post-term birth compared to those with full-time workers at larger companies (with 100 or more employees), public servants, or corporate board members. The aRRs for these groups were 1.53 (95% CI, 1.25 to 1.87), 1.10 (95% CI, 1.05 to 1.17), 1.17 (95% CI, 1.07 to 1.27), and 1.35 (95% CI, 1.14 to 1.59), respectively.

DISCUSSION

This study explored the relationship between post-term birth and neonatal mortality, as well as the factors contributing to post-term birth in Japan. The findings indicated a positive association between post-term birth and neonatal mortality. Additionally, specific factors were identified as predictors of post-term birth. The interpretation of these results, their implications, and the limitations of the study are detailed in this section.

This study demonstrated an elevated risk of neonatal mortality for post-term birth. Prior research has linked such birth to a heightened risk of respiratory and infectious neonatal morbidities, including asphyxia, pneumonia, and septicemia [3,10]. It is plausible that neonatal morbidity resulting from post-term birth contributes to an increased risk of neonatal mortality. While findings on the relationship between postterm birth and neonatal mortality differ across countries [7-11], a key strength of this study is the use of recent data encompassing all births within a single nation.

Regarding the factors associated with post-term birth, this study indicated that female infants had a higher risk of post-term birth than male infants. This contrasts with a study from Sweden, which indicated that male infants faced a higher risk [20]. However, studies from Korea and Ethiopia did not find a higher risk for male compared to female infants [11,21]. In our study, older maternal age was positively associated with postterm birth, which is consistent with findings from a single medical institution in Japan [17]. Similar results have been reported in studies from Sweden and the United States [14,15]. In contrast, a study from China found that younger maternal age was linked to a higher risk of post-term birth [16], indicating that the relationship between maternal age and post-term birth may vary by country. In the present study, the highest rate of post-term birth was observed in the group aged under 20 years. This may be due to factors such as a higher proportion of primiparous females in this younger group. Additionally, maternal primiparity was associated with a higher risk of post-term birth compared to maternal multiparity. Accordingly, studies from Sweden and the United States have found that nulliparity (that is, primiparous pregnant females) is a risk factor for post-term birth [14,15]. Conversely, a study from China reported that primiparous females had a slightly lower risk of post-term birth [16].

Unmarried females were found to be at a comparatively high risk of post-term birth, a finding consistent with a study conducted in Ethiopia [11]. Additionally, females with household occupations other than full-time worker at a larger company tended to face a higher risk of post-term birth than those in households with such a full-time worker. The risk ratio (RR) was relatively high among unemployed households. These findings suggest that socioeconomic status is associated with the risk of post-term birth, with several possible reasons for this association. First, socioeconomic status is linked to the utilization of antenatal care [22,23], and the risk of post-term birth decreases with an increased number of antenatal care visits [16]. Furthermore, females who attend more visits are more likely to receive pregnancy management services more often and undergo labor induction before reaching 42 weeks of gestational age [16]. Second, socioeconomic status is linked to the prevalence of obesity. In Japan, lower educational attainment and household income have been positively associated with overweight and obesity among adult females [24]. A meta-analysis revealed a non-linear association between maternal body mass index and post-term birth [25], with the strength of the association intensifying with increasing body mass index. Among household occupations in the present study, farmers exhibited the highest RR. This finding aligns with a study from the United States, which indicated that newborns from agricultural households were at an elevated risk of post-term birth [26].

Moreover, non-Japanese mothers were at a higher risk of post-term birth than Japanese mothers, aligning with a study conducted at a single medical institution in Japan [18]. This increased risk among foreign mothers has also been observed elsewhere [21,27], with lower utilization of antenatal care potentially contributing to post-term birth [21]. Studies from other countries have indicated that immigrant mothers are more likely to experience delayed or inadequate antenatal care [28,29].

The rate of post-term births has been declining in recent years, with Japan currently experiencing a low incidence. However, the present study reveals differences in risk of post-term birth depending on socio-demographic factors. Notably, certain disadvantaged groups, including unmarried females and those from unemployed households, were more likely to experience post-term birth. Understanding why some sociodemographic groups in Japan face a higher risk of post-term birth is crucial. Future research should explore the variations in physical characteristics and health-related behaviors during pregnancy that may relate to socio-demographic factors. In Japan, as well as globally, labor is often induced before reaching 42 weeks of gestation [30-32]. Therefore, it is possible that females exhibiting a higher risk of post-term birth are relatively unlikely to seek medical care during pregnancy. Consequently, it is beneficial to recommend antenatal care for these high-risk females, particularly if they are otherwise less inclined to receive such care than other females. Antenatal care allows pregnant patients to accurately understand their expected delivery dates. Moreover, it ensures regular monitoring of their health and that of their fetuses by an obstetrician, which can help reduce the incidence of post-term births. In Japan, the costs for antenatal care (prenatal check-ups) are currently subsidized by public funds for up to 14 visits [33]. Despite this, some pregnant females, such as non-Japanese females and those with lower socioeconomic status, may face challenges in accessing antenatal care. If the uptake of this care differs based on the characteristics of pregnant females, additional administrative measures may be required to support high-risk individuals.

The present study had several limitations. First, we lacked access to data on certain factors, such as maternal body mass index, educational attainment, and the utilization of antenatal care, all of which have been associated with post-term birth in other countries. Moreover, we lacked information on labor induction, which could influence the incidence of post-term birth. Therefore, future research in Japan should consider these factors. Second, it was unclear whether the gestational age for each infant was estimated using the ultrasound method or the last menstrual period method. Despite these limitations, this study is the first in Japan to explore the association between post-term birth and neonatal mortality using recent nationwide data. Furthermore, through the analysis of data from the Vital Statistics of Japan, the results represented the overall pattern of the associations of post-term birth with neonatal mortality and birth characteristics in Japan.

Supplemental Materials

Notes

Conflict of Interest

The authors have no conflicts of interest associated with the material presented in this paper.

Funding

This study was supported by JSPS KAKENHI under Grant No. JP22K17372.

Author Contributions

Conceptualization: Okui T. Data curation: Okui T. Formal analysis: Okui T. Methodology: Okui T. Funding acquisition: Okui T. Writing – original draft: Okui T. Writing – review & editing: Okui T, Nakashima N.

Acknowledgements

Figure. 1.

Flowchart of study population selection.

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References

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