This observational study was conducted retrospectively using data extracted from medical records. The study participants were term infants (born between 37 weeks 0 days of gestation and 41 weeks 6 days of gestation) born between 2015 and 2022 at the Tokyo Metropolitan Toshima Hospital, a secondary hospital in the Tokyo Metropolitan area that manages deliveries performed after 35 weeks of gestational age. All birth data between the study periods were extracted, and individual data on gestational age, sex, birth weight, maternal age, Apgar score, cesarean section (yes/no), and hospitalization after birth (yes/no, and if yes, reason for hospitalization) were extracted anonymously. ‘Hospitalization’ was defined as patients admitted to the growth care unit of the pediatric ward. The pediatrician performed examinations and made decisions whenever the attending gynecologist requested a consultation.

Gestational weeks were calculated based on the recommendations of the Japan Association of Obstetricians and Gynecologist in Japan. Firstly, if there is precise information regarding fertilization, including data on basal body temperature, limited intercourse, artificial insemination, or in vitro fertilization, the test day was classified as 2 weeks 0 days of gestation. Secondary, if the menstrual cycle was regular and lasted from 28 to 30 days and if the date of the last menstrual period was known accurately, the first day of the last menstrual period was considered 0 week 0 day of gestation. If neither of the following information was available, a crown-rump length of 15–30 mm or a biparietal diameter of 20–30 mm was used to determine gestational age.

The reasons for hospitalization and their criteria were as follows: neonatal jaundice (jaundice that requires phototherapy), feeding disorder (mal-feeding requiring hospitalization), transient tachypnea of the newborn (respiratory disorder without pneumonia, pneumothorax), meconium aspiration syndrome (respiratory disorder with meconium aspiration), hypoglycemia (blood glucose less than 40 mg/dl that does not improve with feeding), acute gastric mucosal lesions (bloody vomiting), melena bloody stools), asphyxia (diagnosed by attending pediatrician and requiring hospitalization), vomiting (vomiting requiring management), neonatal fever (significant fever over 38°C), and, apnea (respiratory arrest for more than 30 s or complicates bradycardia). To diagnose respiratory disorders, we used chest x-ray results, which were examined by a blinded pediatrician, and transient tachypnea and meconium aspiration syndrome were clearly discriminated. Pneumonia was diagnosed based on consolidation on radiographs.

First, the participants were limited to term infants based on gestational age data. Early term was defined as birth between 37 weeks, 0 days, and 38 weeks, and 6 days of gestational age.

Comparisons between early term (born before 38 weeks) and other term infants (born after 38 weeks and 0 days of gestation) were performed. The same procedure was then performed separately for the groups of infants born in the 37th and 38th weeks of gestation.

When analyzing the participants’ characteristics, the t-test was used for continuous variables and the chi-square test was used for categorical variables. Logistic regression was used to analyze the association between hospitalization (yes or no) and each reason for hospitalization, and early term or 37 and 38 weeks of gestational age were analyzed separately. Statistical analyses were performed using Stata software version 15.1 (Stata Corp, 4905 Lakeway Drive College Station, TX 77845, USA). Statistical significance was set at p < 0.05.

This study was approved by the Ethics Committee of Tokyo Metropolitan Toshima Hospital (approval number: Jin 4–17). The requirement for patient consent was waived owing to the use of anonymous data.

Among the 4,821 infants born during the study period, 151 were preterm (born before 37 weeks of gestational age) and one was born after 42 weeks of gestational age. After excluding these infants, the remaining 4,669 infants were included in the present study. Among these, 463 (9.9%) were born at 37 weeks, 1,270 (27.2%) at 38 weeks, 1,400 (29.9%) at 39 weeks, 1,189 (25.4%) at 40 weeks, and 347 (7.4%) at 41 weeks of gestation. The total number of early term infants, defined as those born between 37 and 38 weeks of gestational age, was 1,733 (37.1%).

The present study showed complicated clinical course in infants born in the early term compared with those born after 38 weeks of gestation. The former group had higher odds ratios for hospitalization, apnea, and hypoglycemia. Further analysis distinguishing between early term infants born at 37 and 38 weeks gestation revealed more complicated clinical outcomes in infants born at 37 weeks gestation.

Overall, the results of this study show that early term infants have complicated neonatal outcomes, including higher risks of hospitalization, apnea, and hypoglycemia, compared to infants born after 39 weeks of gestation. Among early term infants, those born at 37 weeks of gestation have a worse outcome than those born at 38 weeks of gestation.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving humans were approved by the Ethics Committee of Tokyo Metropolitan Toshima Hospital. The studies were conducted in accordance with the local legislation and institutional requirements. Written informed consent for participation was not required from the participants or the participants’ legal guardians/next of kin in accordance with the national legislation and institutional requirements.

TK: Formal Analysis, Investigation, Resources, Visualization, Writing – original draft. KT: Data curation, Funding acquisition, Investigation, Resources, Validation, Visualization, Writing – original draft. YM: Conceptualization, Funding acquisition, Investigation, Project administration, Visualization, Writing – original draft. DY: Data curation, Methodology, Software, Validation, Writing – review & editing. TN: Resources, Validation, Visualization, Writing – review & editing. KS: Data curation, Funding acquisition, Investigation, Validation, Writing – review & editing. HS: Conceptualization, Project administration, Supervision, Writing – review & editing.

The author(s) declare financial support was received for the research, authorship, and/or publication of this article. This work was supported by research fund from the Tokyo Metropolitan Toshima Hospital. (No grant number).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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