In the present study, we explored the trends in PTB and LBW in a perinatal center in Japan, and examined the contributing factors and changes in neonatal outcomes. Although term-LBW did not increase, PTB, in particular medically indicated PTB, increased considerably. These increases were largely explained by changes in obstetric interventions. Despite the increases, the neonatal outcomes did not worsen, and instead the Apgar scores and proportions of NICU admission improved.
The proportions of PTB and LBW increased with peaks around 2005 in the present study, consistent with the national vital statistics in Japan (4.1% and 5.2% in 1980 to 5.7% and 9.6% in 2010, respectively)
]. In contrast, term-LBW did not increase, indicating that the observed increase in LBW can probably be explained by the increase in earlier deliveries (i.e., PTB). The finding that the increase in medically indicated PTB was larger than that in spontaneous PTB was consistent with previous studies in western countries
]. Although the increase in PTB in western countries has been almost entirely among the late preterm (34 to 36 weeks)
], the increase in PTB at less than 33 weeks also accounted for a small fraction of the increase in total PTB in the present study, consistent with the national vital statistics in Japan. For example, about 14% of the increase in total PTB was explained by the increase in PTB at equal to or less than 31 weeks in Japan
The increase in PTB was largely explained by changes in obstetric interventions, in particular caesarean sections, followed by changes in maternal sociodemographic and behavioral factors. Indeed, the proportion of caesarean sections has been steadily rising in Japan (14.7% for hospitals and 9.9% for clinics in 1996 to 23.3% and 13.0% in 2008, respectively)
]. Although the changes in sociodemographic and behavioral factors may directly contribute to the rise in the proportion of caesarean sections, obstetricians may also be prone to conduct caesarean sections in a more proactive manner, independent of such demographic changes. In Japan, people consider that the lean structure of women and reduced weight gain during pregnancy might contribute to the rise in LBW
], because low BMI and poor weight gain are risk factors for LBW
]. However, in the present study, obese mothers increased and mothers recently showed more adequate weight gain, and thus the increases in LBW as well as PTB could not be explained by these factors.
Despite the increases in PTB and LBW, the neonatal outcomes did not worsen, and instead showed improvement. Previous studies also indicated the benefits of obstetric interventions on perinatal and neonatal mortality
]. The present study supported these previous studies by providing further findings for the benefits on biological indicators (Apgar score and cord blood pHs). Accordingly, it can be inferred that obstetricians may intervene appropriately (i.e., conduct necessary interventions at appropriate periods), which may contribute to low infant mortality and neonatal mortality rates (2.29 and 1.09 per 1000 births in 2010, respectively) in Japan
], in combination with advancement of NICU facilities. However, it should be noted that, since the prognostic outcomes of late preterm births are reported to be worse than those of term newborns in general
], further research should be conducted to evaluate whether such obstetric interventions lead to better prognostic outcomes in newborns
One of the strengths of the present study is that we could utilize the biological indicators from a clinical dataset, compared with the previous studies using birth certificates. On the other hand, there is a problem with generalizability of the findings. As described in the Methods section, not all of the babies in the western part of Shizuoka are born in this particular hospital. In addition, the hospital is the main perinatal center in the area, and therefore manages not only low-risk but also high-risk deliveries. As a consequence, the proportions of PTB and LBW were higher than those reported nationally, as expected. Thus, it is possible that this hospital-based sampling method may affect the external validity. However, the present findings are consistent with previous studies based on general populations conducted in western countries, and thus the main findings would not be affected considerably.
The methods for measuring gestational weeks, obtaining Apgar scores, and evaluating blood gas were standardized throughout the study period. Moreover, the standard of NICU admission in this institution did not change. Thus, changes in obstetric diagnoses or techniques during the study period may not affect the present findings.
We could not obtain individual socioeconomic status variables (education or income) other than occupation. Therefore, other factors that we did not consider in the analyses might explain the discrepancy to some extent. However, since we adjusted for various variables that may define parental socioeconomic status or other variables, the discrepancy generated from these factors would not be larger than that for caesarean sections.