What are the differences in outcomes between being born at 38 weeks of gestation versus 40 weeks of gestation?

From the Guidelines

Being born at 38 weeks versus 40 weeks generally results in similar outcomes for most babies, with 38 weeks considered early term and 40 weeks being the traditional due date. Both are considered full-term births, with babies born at 38 weeks may have a slightly higher risk of minor respiratory issues compared to those born at 40 weeks, as the final weeks of pregnancy allow for continued lung maturation 1. However, most 38-week babies are developmentally ready for life outside the womb with fully developed organs. The birth weight difference is typically small, with 38-week babies weighing slightly less on average. Breastfeeding success rates and cognitive development outcomes are generally comparable between the two gestational ages.

For planned deliveries without medical necessity, waiting until 39 weeks is often recommended to give the baby the most complete development time, as supported by the ARRIVE trial which found no statistically significant difference in primary perinatal outcomes between elective induction of labor and expectant management at 39 weeks of gestation 1. However, spontaneous labor at 38 weeks is not typically concerning, and many healthcare providers consider births between 38-42 weeks to be within the normal range for healthy outcomes. It's worth noting that the American Journal of Obstetrics and Gynecology recommends against the use of low-molecular-weight heparin for the sole indication of prevention of recurrent fetal growth restriction, and suggests delivery at 38-39 weeks of gestation with fetal growth restriction when the estimated fetal weight is between the 3rd and 10th percentile and the umbilical artery Doppler is normal 1.

Some key points to consider include:

• Babies born at 38 weeks may have a slightly higher risk of minor respiratory issues
• Birth weight difference is typically small between 38-week and 40-week babies
• Breastfeeding success rates and cognitive development outcomes are generally comparable between the two gestational ages
• Waiting until 39 weeks is often recommended for planned deliveries without medical necessity
• Spontaneous labor at 38 weeks is not typically concerning, and many healthcare providers consider births between 38-42 weeks to be within the normal range for healthy outcomes.

From the Research

Comparison of Birth at 38 Weeks vs 40 Weeks

• Being born at 38 weeks vs 40 weeks can have different implications for the health of the infant, particularly in terms of respiratory distress syndrome (RDS) and the need for surfactant therapy 2, 3, 4, 5, 6.
• Studies have shown that preterm infants, including those born at 38 weeks, are at a higher risk of developing RDS and may require surfactant therapy to reduce the risk of death and bronchopulmonary dysplasia 2, 3, 5.
• The use of non-invasive respiratory support, such as continuous positive airway pressure (CPAP), has been shown to be a safe and effective approach for preterm infants with RDS, and may reduce the need for intubation and surfactant therapy 2, 3, 4, 6.
• A study published in 2024 found that the respiratory severity score (RSS) during the first 3 hours of life can be used to predict the failure of non-invasive respiratory support and the need for late rescue surfactant administration in preterm infants 4.
• Another study published in 2014 found that treatment with early CPAP rather than intubation/surfactant is associated with less respiratory morbidity by 18-22 months corrected age 6.

Surfactant Therapy

• Surfactant therapy has been shown to be effective in reducing the risk of death and pulmonary air leaks in preterm infants with RDS 2, 3, 5.
• Animal-derived surfactants are currently the treatment of choice, but synthetic surfactants are being developed and may offer a comparable efficacy profile 3, 5.
• The optimal timing, dose, and method of surfactant administration are still being studied, but early administration has been shown to reduce the risk of RDS and its complications 3, 4, 5.

Respiratory Outcomes

• Studies have shown that preterm infants who receive surfactant therapy or non-invasive respiratory support may have improved respiratory outcomes, including reduced risk of wheezing, cough, and respiratory illnesses diagnosed by a doctor 4, 6.
• Longitudinal assessment of pulmonary morbidity is necessary to fully evaluate the potential benefits of respiratory interventions for neonates 6.