Babies’ lungs are usually developed enough for normal breathing by around 37 weeks of pregnancy, but microscopic structures keep maturing through early childhood and are not truly “finished” until roughly age 8.

Key timeline in the womb

  • Around 24–28 weeks: Many babies born this early can sometimes breathe with intensive medical support because basic air sacs and some surfactant (the “slippery” substance that keeps air sacs open) are present.
  • Around 28–32 weeks: Lung structures and surfactant production improve, so survival and breathing outcomes get much better, though these babies are still premature and often need support.
  • Around 37 weeks (term): The lungs are considered functionally mature for life outside the womb in most babies, and the risk of serious breathing problems drops compared with earlier births.

After birth: still developing

  • From birth to about 2–3 years: The number of tiny air sacs (alveoli) increases sharply, making lungs much more efficient at gas exchange.
  • Up to about 8 years old: Lung size and structure continue to grow and remodel; experts describe alveolar development and lung growth as ongoing through “early childhood,” often cited up to school age.

Premature babies’ lungs

  • Babies born before 37 weeks may have immature lungs and lower surfactant levels, so they are more prone to breathing problems like respiratory distress syndrome and may need oxygen, ventilation, or medications.
  • Even after discharge, ex‑preterm children can have more respiratory illnesses in infancy and toddler years, though many gradually “catch up” as lung growth continues.

Simple takeaway for parents

  • “When are babies’ lungs fully developed?”
    • Functionally ready for normal breathing: around 37 weeks of pregnancy in most babies.
* Structurally and microscopically “finished”: continuing to mature all the way through early childhood, roughly up to about 8 years old.

If there are concerns about lung development (such as threatened preterm birth or a baby with ongoing breathing issues), a pediatrician or maternal–fetal medicine specialist can give specific, individualized guidance.

Information gathered from public forums or data available on the internet and portrayed here.