Embryology · Fetal period, placenta and twins
Fetal lung, kidney and brain maturation: what preterm birth interrupts !
- Topic
Late in pregnancy the lungs, brain, kidneys, gut and eyes mature. Being born early cuts this short and explains the problems of preterm babies.
Updated 2026-11-01
What it is
The third trimester is a time of maturation: the lung makes surfactant, the kidney finishes forming nephrons, the germinal matrix of the brain regresses and the retina finishes its blood supply. Preterm birth leaves these processes half done. Each typical complication of prematurity matches an organ that had not finished maturing.
Key facts
- Type II pneumocytes start making surfactant around weeks 20 to 24; the amount rises steeply in the last weeks.
- Brown fat, key for making heat, builds up late in pregnancy: preterm babies lose heat easily.
- Coordinated sucking and swallowing appear around weeks 32 to 34.
- Myelination starts in the fetal period and goes on for years after birth.
What matures late in pregnancy and what happens if it is cut short
| Organ | What matures | If born early |
|---|---|---|
| Lung | Terminal sacs and surfactant | Respiratory distress |
| Brain | Regression of the germinal matrix | Intraventricular haemorrhage |
| Gut | Motility, barrier and defences | Necrotizing enterocolitis |
| Retina | Retinal vessels out to the periphery | Retinopathy of prematurity |
| Ductus arteriosus | Closing response to oxygen | Patent ductus arteriosus |
| Kidney | Last nephrons | Fewer nephrons for life |
Clinical notes
- Any birth before week 37 of gestation counts as preterm.
- Periventricular leukomalacia in preterm infants is linked to cerebral palsy.
- Too much oxygen promotes retinopathy of prematurity and bronchopulmonary dysplasia.
- Prematurity is one of the leading causes of newborn death in Colombia and worldwide.
Test yourself
Think of the answer, then tap the question to see it.
By about which week is there usually enough surfactant to breathe without difficulty?
By about weeks 34 to 35 of gestation; before that the risk of respiratory distress syndrome rises.
Which brain structure in preterm infants explains intraventricular haemorrhage?
The subependymal germinal matrix, highly vascular and fragile, which regresses late in pregnancy.
Why are preterm infants more prone to infection?
Because most maternal IgG crosses the placenta in the third trimester and their own immunity is still immature.
What is given to the mother to speed fetal lung maturation when preterm birth is expected?
Antenatal corticosteroids, which increase surfactant synthesis and also reduce intraventricular haemorrhage.
Related
- Fetal period: month 3 to birth
- Surfactant and fetal lung maturity
- Lung development: the five stages
- Fetal circulation and the changes at birth
- Patent ductus arteriosus
- Kidney: pronephros, mesonephros, metanephros and ascent
- Fetal growth and growth restriction
Related conditions
- Preterm labour and birth
- Neonatal respiratory distress syndrome (hyaline membrane disease)
- Prematurity and its complications
- Necrotising enterocolitis
- Patent ductus arteriosus
- Cerebral palsy
Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.
Sources: Neonatal respiratory distress syndrome: MedlinePlus Medical Encyclopedia (MedlinePlus, U.S. National Library of Medicine, 2025); Fetal development: MedlinePlus Medical Encyclopedia (MedlinePlus, U.S. National Library of Medicine, 2025); Langman's Medical Embryology, 13th ed., ch. 8: Third Month to Birth: The Fetus and Placenta (Wolters Kluwer, 2015).
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Representative model, for educational purposes only.