Protecting Tiny Lungs at 34-36 Weeks
Imagine this: A baby arrives just three weeks early, seemingly strong and well-developed. Yet, this infant faces a hidden vulnerability—immature lungs struggling to take their first breaths. This is the reality for late preterm infants, born between 34 and 36 weeks of gestation. While they represent the majority (roughly 70%) of all preterm births, their unique challenges have only recently come into sharp focus 3 .
Babies born between 34-36 weeks gestation account for about 70% of all preterm births.
Even at 34-36 weeks, lungs may not be fully mature, leading to respiratory complications.
For infants born before 34 weeks, the evidence is unequivocal: a single course of corticosteroids given to the mother dramatically reduces the risk of neonatal death, respiratory distress syndrome (RDS), intracranial hemorrhage, and necrotizing enterocolitis 1 . These steroids accelerate fetal lung maturation by boosting the production of surfactant, a critical substance that keeps the tiny air sacs in the lungs from collapsing. The benefits are so profound that administering ACS is considered one of the most important advances in perinatal medicine 1 5 .
The question of steroid efficacy in the late preterm period demanded rigorous evidence. The pivotal answer came from the Antenatal Late Preterm Steroids (ALPS) trial, a major randomized controlled trial published in 2016 in the New England Journal of Medicine 4 6 .
2,831 pregnant women with singleton pregnancies between 34 weeks 0 days and 36 weeks 6 days gestation at high probability of delivery within 7 days 4 .
Two 12-mg intramuscular injections of betamethasone or placebo, given 24 hours apart 4 .
Need for respiratory support within 72 hours of birth (CPAP, oxygen, mechanical ventilation, or ECMO) 4 .
| Outcome | Betamethasone Group | Placebo Group | Relative Risk | NNT/NNH |
|---|---|---|---|---|
| Need for Respiratory Support | 11.6% | 14.4% | 0.80 | NNT=35 |
| Severe Complications | 8.1% | 12.1% | 0.67 | NNT=25 |
| Transient Tachypnea (TTN) | 6.7% | 9.9% | 0.68 | NNT=31 |
| Neonatal Hypoglycemia | 24% | 15% | 1.6 | NNH=11 |
Alternative synthetic glucocorticoid. Also crosses placenta effectively. Used in some studies; debate exists over comparative effectiveness/safety vs. betamethasone 1 .
Inert substance identical in appearance to active drug. Serves as the control. Control intervention in ALPS trial and other RCTs to isolate true effect of steroid 4 .
Delivers continuous positive airway pressure to keep airways open. Measures primary outcome (respiratory support). Defined "need for respiratory support" in ALPS (≥2h use) 4 .
The story of antenatal corticosteroids for late preterm infants exemplifies the dynamic nature of medical evidence. The ALPS trial was a game-changer, demonstrating that the lung-maturing benefits of steroids extend meaningfully into the 34-36 week window, offering significant protection against respiratory morbidity for these vulnerable "near-term" babies. This evidence has rightfully shifted guidelines, making ACS a recommended option for carefully selected singleton pregnancies at high, imminent risk of late preterm delivery.
"The decision to use late preterm steroids hinges on a nuanced risk-benefit calculation for each individual pregnancy,"