Trends in gestational age and short-term neonatal outcomes in the United States

Preterm and early-term births have been steadily rising in the United States. Data from the National Vital Statistics System shows that between 2014 and 2022, the preterm birth rate (<37 weeks gestation) increased by 12 %. Within this category, early preterm births (<34 weeks) rose by 4 %, while late preterm births (34–36 weeks) increased by 15 %. Early-term births (37–38 weeks) saw a 20 % rise, whereas births at 39–40 weeks and ≥41 weeks decreased by 6 and 28 %, respectively [1]. These trends reflect a gradual shift toward earlier gestational ages at birth. However, there is limited data on how these changes impact short-term neonatal outcomes. We hypothesized that the shift in gestational age distribution would lead to an increase in newborn respiratory distress, Neonatal intensive care unit (NICU) admissions, and antibiotic exposure. The primary objective of this study was to evaluate the population-based temporal trends in these outcomes in the U.S. between 2016 and 2023.

We conducted a retrospective, repeated cross-sectional analysis of the Natality dataset from the Wide-ranging Online Data for Epidemiologic Research (WONDER) from the Centers for Disease Control and Prevention (CDC) from 2016 through 2023 [2]. This dataset reports statistics for live births in the United States to U.S. residents based on the 2003 revised birth certificate. All states in the US have been using the 2003 revised birth certificate since 2015. The data pertains to all live births (stillbirths excluded) and are available by a variety of demographic characteristics, such as the mother’s race, and mother’s age, and health and medical items, such as tobacco use, and method of delivery.

Six abnormal conditions of the newborn are separately identified in a checkbox format: 1) assisted ventilation required immediately following delivery; 2) assisted ventilation required for more than 6 h; 3) NICU admission; 4) newborn given surfactant replacement therapy; 5) antibiotics received by the newborn for suspected neonatal sepsis; and 6) seizure or serious neurological dysfunction. This item allows for the reporting of more than one condition and includes a choice of “None of the above”. If the item is not completed (i.e., none of the boxes are checked), it is classified as “Not stated” and this is usually <1 % [3]. The data are derived from birth certificates issued in the years 2016–2023 and more than 99 % of births occurring in the United States are registered [3]. The data on these abnormal newborn conditions are obtained directly form the medical record using a worksheet provided by the CDC. The exposure variable was calendar year, and the outcomes were the rate of NICU admissions, assisted ventilation, surfactant administration, and exposure to antibiotics, expressed as per 1,000 live births. Additionally, we examined the changes in these outcomes in the context of any changes in maternal diagnosis of chorioamnionitis and maternal receipt of antibiotics. The CDC defined chorioamnionitis based on clinical grounds or maternal temperature >38 °C.

Trend analysis was performed using linear regression, with a p-value <0.05 considered statistically significant. The strength of the association between the calendar year and the outcomes was assessed with correlation coefficient (R). This cross-sectional study did not require institutional review board approval or patient informed consent because it used publicly available deidentified data in accordance with Common Rule 45 CFR § 46.

Out of 29, 874, 337 live births from 2016 to 2023, 51.1 % were male, 32 % were delivered via cesarean section, and 97.5 % had a 5-min Apgar score ≥7. The average gestational age and birthweight were 38.4 weeks and 3,254 gm, respectively. Diabetes, hypertension, and twin or higher order births complicated 8.3 , 10.5, and 1.4 % of pregnancies, respectively.

Between 2016 and 2023, the average gestational age at birth decreased from 38.5 to 38.3 (p<0.01, R=0.97) while average birth weight decreased from 3,267 to 3,240 gm (p<0.001; R=0.97). NICU admission rates (per 1,000 livebirths) increased from 87 to 98 (p<0.001, R=0.81). The overall rate of immediate assisted ventilation increased from 39.2 to 63.2 (p<0.001, R=0.99) while assisted ventilation >6 h increased from 13.7 to 20.9 (p<0.001; R=0.99) [Figure 1]. The rate of antenatal steroid administration also increased from 23.8 to 38.5 (p<0.01, R=0.72) but there was no significant change in surfactant replacement therapy (4.83–5.04; p=0.19). Neonatal exposure to antibiotics for suspected sepsis declined from 23 to 19 (p<0.001; R=0.95) despite an increase in maternal chorioamnionitis from 15.3 to 17.6 per 1,000 (p=0.02; R=0.60) [Figure 2]. However, there was no significant change in the rate of antibiotic administration for maternal chorioamnionitis (9.9–10.9; p=0.2; R=0.49).

Figure 1:

Trends in the rate of maternal receipt of antenatal steroids, assisted ventilation >6 h, and surfactant replacement therapy in the United States from 2016 to 2023.

Figure 2:

Trends in maternal chorioamnionitis, antibiotics for maternal chorioamnionitis, and neonatal antibiotic exposure for suspected sepsis.

Between 2016 and 2023, there were significant declines in gestational age and birth weight, coupled with increases in NICU admissions and respiratory morbidity. These trends point to a potential increase in healthcare resource utilization. The shifts in GA and birth weight align with the results of previous studies [4]. These are due to changes in obstetric practices such as increases in induction of labor and cesarean deliveries at earlier gestations [4]. The increase in respiratory morbidity without a corresponding increase in surfactant replacement therapy suggests that this may be due to increased respiratory distress in late preterm or term babies that typically do not require surfactant therapy replacement. Another reason could be an increase in respiratory conditions like retained fetal lung fluid syndrome (previously called transient tachypnea of the newborn) which is not usually treated with surfactant replacement therapy. Alternatively, this observation could also due to an increasing shift towards non-invasive respiratory support modalities especially in preterm newborns in the US [5]. The decline in neonatal antibiotic exposure may reflect improved infectious disease management and judicious use of antibiotics emanating from enhanced antibiotic stewardship [6], [7], [8]. The Kaiser Early Onset Sepsis calculator was developed and implemented in 2016 hoping to target antibiotic use depending on individualized risk assessment and this could have contributed to the reduced use of antibiotics in newborns observed in this study [9], 10]. Additionally, the American Academy of Pediatrics published updated Early Onset Sepsis guidelines in 2018 and these provided a framework for the development of evidence-based approaches to sepsis risk assessment and management in newborn infants [11], 12]. This could have also led to the decreased antibiotic use for suspected sepsis observed in this study.

The strength of this study lies in the use of a national dataset which makes the findings nationally representative. However, the findings of the study must be interpreted within the limitations of the study. The limitations include the use of information documented on birth certificates. Events such as respiratory distress or suspected sepsis that occurred after the birth certificate worksheet had been completed would not be captured in the dataset. Furthermore, previous research has demonstrated that while birth certificates have high reliability, validity and sensitivity for GA and BW, the sensitivity for labor complications such as maternal fever and other neonatal complications such as NICU admission and assisted ventilation are low [13]. Thus, it is likely that the data reported in the present underestimates the true scope of these short-term neonatal outcomes. Given the scope of this population-based study, which encompasses approximately 30 million births, even small shifts can produce statistically significant trends that may not be clinically relevant. Finally, we sought to determine the association between shifts in GA and the temporal trends in short-term neonatal outcomes and therefore did not perform any regression analysis. Thus, these trends do not establish causality. Further research is needed to validate these findings as well as identify the underlying drivers of these changes.