At what week of gestation are the lungs fully developed?

Lung Development Timeline in Pregnancy

Fetal lungs reach full maturity between 35-36 weeks of gestation, with the alveolar phase beginning around 28 weeks and continuing past birth.

Stages of Lung Development

Lung development follows a series of orchestrated events that are critical for proper respiratory function after birth:

1. Embryonic Period (up to 7 weeks gestation)

   • Initial lung bud formation
   • Early bronchial development

2. Pseudoglandular Period (7-16 weeks gestation)

   • Formation of conducting airways
   • Development of bronchial tree

3. Canalicular Period (16-26 weeks gestation)

   • Bronchial elements actively dividing
   • Increased vascularity
   • In humans, extends to about week 20 1

4. Saccular Phase (26-36 weeks gestation)

   • Alveoli first appear around 24-28 weeks
   • Significant increase in gas exchange surface area
   • Critical for viability outside the womb

5. Alveolar Phase (36 weeks to early childhood)

   • Begins around 36 weeks and continues after birth
   • Maturation of alveoli for efficient gas exchange
   • Development continues into early childhood

Clinical Significance

Preterm Birth and Lung Maturity

• Before 34 weeks: Significant risk of respiratory distress syndrome due to immature lungs and insufficient surfactant production

• 34-36 weeks: Lungs approaching maturity but still at higher risk of respiratory complications compared to term infants 2

• After 36-37 weeks: Lungs generally considered functionally mature with adequate surfactant production 1

Antenatal Corticosteroids

• Guidelines recommend antenatal corticosteroids for women at risk of preterm birth before 37 weeks to accelerate fetal lung maturation 1

• Most effective when administered between 24 hours and 7 days before delivery

• For late preterm births (34 0/7 to 36 6/7 weeks), a single course of antenatal corticosteroids is recommended for those at high risk of delivery within 7 days 1

Important Considerations

• Despite documented lung maturity via testing (e.g., lecithin/sphingomyelin ratio), neonates delivered at 36-38 weeks still have higher rates of adverse respiratory outcomes compared to those delivered at 39-40 weeks 3

• Fetal lung volume measurements show that fetuses delivered very preterm (<32 weeks) have reduced lung volumes compared to those born at term, suggesting antenatal factors may contribute to pulmonary morbidity 4

• Lung development continues after birth, with alveolarization continuing through early childhood

Clinical Implications

• Elective deliveries should be avoided before 39 weeks unless there is documentation of fetal lung maturity

• When delivery before 37 weeks is anticipated, antenatal corticosteroids should be considered to accelerate lung maturation

• Even with documented lung maturity, infants born before 39 weeks have higher risks of respiratory complications than those born at 39-40 weeks

• For women with placenta accreta spectrum requiring planned delivery, the window of 34 0/7-35 6/7 weeks is suggested as the preferred gestational age for scheduled delivery, balancing fetal lung maturity with maternal risks 1