[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Reproduction, pregnancy and the fetus](https://durgical.com/en/physiology/#s-repro) › Newborn physiology and fetal maturation

[Physiology](https://durgical.com/en/physiology/) · [Reproduction, pregnancy and the fetus](https://durgical.com/en/physiology/#s-repro)

# Newborn physiology and fetal maturation

- Concept
- Exam priority: medium

At birth the baby must breathe, keep warm, feed and clear bilirubin on its own; the lungs, liver and kidneys are still maturing.

Updated 2026-11-01

Also called: Neonatal adaptation, Transition to extrauterine life

**See it in 3D:** [Liver](https://durgical.com/en/anatomy/liver/) · [Right lower lobe](https://durgical.com/en/anatomy/inferior-lobe-of-right-lung/) · [Left lower lobe](https://durgical.com/en/anatomy/inferior-lobe-of-left-lung/) · [Thyroid gland](https://durgical.com/en/anatomy/thyroid-gland/) · [Skin](https://durgical.com/en/anatomy/skin/)

## Key points

- Surfactant (type II pneumocytes) is adequate from about 34–35 weeks; antenatal glucocorticoids speed its production.
- Physiological jaundice: high red cell mass, short HbF red cell life and immature UGT1A1; appears after 24 h and peaks at 3–5 days in term babies.
- Jaundice in the first 24 h is never physiological (think hemolysis, for example Rh or ABO incompatibility).
- Newborns lose heat fast (large surface-to-mass ratio) and make heat by non-shivering thermogenesis in brown fat (UCP1).
- Vitamin K stores are low (poor placental transfer, sterile gut, little in breast milk): hence routine vitamin K at birth.
- Newborn screening (in Colombia it includes TSH for congenital hypothyroidism) detects treatable disorders before symptoms.

## What it is

In minutes the newborn switches from placental to pulmonary gas exchange, then over days takes over temperature control, glucose supply, bilirubin clearance and water balance. Several organs are immature at term and more so in preterm babies, which explains the classic neonatal problems: respiratory distress, jaundice, hypoglycemia, hypothermia and vitamin K deficiency bleeding.

## Key facts

- The first breaths need high negative pressure to open fluid-filled alveoli; lung fluid is cleared by sodium channels (ENaC) and labor catecholamines.
- Glucose falls in the first hours after the placental supply stops; glycogenolysis and gluconeogenesis take over.
- Kidneys concentrate urine poorly (urine osmolality up to about 600–700 mOsm/kg), so newborns are vulnerable to dehydration and to fluid overload.
- Hemoglobin is high at birth (≈14–20 g/dL) and falls to a physiological nadir at about 8–12 weeks as HbF is replaced by HbA.
- IgG crossed from the mother protects the baby for months; IgA arrives in colostrum.

## Reference values (term newborn)

| Parameter | Value | Note |
| --- | --- | --- |
| Heart rate | ≈110–160 /min | |
| Respiratory rate | ≈30–60 /min | |
| Axillary temperature | ≈36.5–37.5 °C | |
| Hemoglobin at birth | ≈14–20 g/dL | higher at altitude |
| Weight loss in the first days | ≤7–10% | birth weight regained by 10–14 days |

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

## Immature system and classic problem

| System | Why immature | Problem |
| --- | --- | --- |
| Lung | Little surfactant before ≈34 weeks | Neonatal respiratory distress syndrome |
| [Liver](https://durgical.com/en/conditions/neonatal-jaundice/) | Low UGT1A1, high bilirubin load | Jaundice; kernicterus if very high |
| [Coagulation](https://durgical.com/en/conditions/vitamin-k-deficiency-bleeding-of-the-newborn/) | Low vitamin K–dependent factors | Vitamin K deficiency bleeding |
| Temperature | Thin skin, large surface, little fat (preterm) | Hypothermia |
| Metabolism | Small glycogen stores | Hypoglycemia (worse in infants of diabetic mothers) |

## Clinical notes

- Infant of a diabetic mother: fetal hyperinsulinism from maternal hyperglycemia causes macrosomia, delayed surfactant and hypoglycemia after cord clamping.
- Breastfeeding jaundice (poor intake, first week) differs from breast milk jaundice (milk factors, after the first week, benign).
- Neonatal polycythemia is more frequent at high altitude and in delayed cord clamping; it raises the bilirubin load.

## Memory aid

Breathe, Warm, Feed, Clear: the four jobs the newborn takes over from the placenta.

## Test yourself

Think of the answer, then tap the question to see it.

## Why do almost all term newborns become slightly jaundiced on day 2–4?

They have a high red cell mass with short-lived HbF cells (more bilirubin) and immature UGT1A1 in the liver (less conjugation), plus enterohepatic recirculation.

## Why are newborns given vitamin K at birth?

Little vitamin K crosses the placenta, breast milk contains little and the gut has no bacteria yet, so factors II, VII, IX and X are low and bleeding can occur.

## How does a newborn produce heat without shivering?

Brown fat mitochondria express UCP1, which uncouples oxidative phosphorylation so energy is released as heat; it is activated by noradrenaline via β3 receptors.

## Why does a preterm baby born at 30 weeks often develop respiratory distress?

Surfactant production is insufficient before about 34 weeks, so surface tension is high and alveoli collapse at end-expiration.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/newborn-physiology-and-fetal-maturation/)

## See it in 3D

The structures of Durgical's 3D anatomy where it happens.

- [Liver](https://durgical.com/en/anatomy/liver/)
- [Right lower lobe](https://durgical.com/en/anatomy/inferior-lobe-of-right-lung/)
- [Left lower lobe](https://durgical.com/en/anatomy/inferior-lobe-of-left-lung/)
- [Thyroid gland](https://durgical.com/en/anatomy/thyroid-gland/)
- [Skin](https://durgical.com/en/anatomy/skin/)

## Related topics

- [Fetal circulation and the transition at birth](https://durgical.com/en/physiology/fetal-circulation-and-the-transition-at-birth/)
- [Surface tension, surfactant and Laplace's law](https://durgical.com/en/physiology/surface-tension-surfactant-and-laplace-s-law/)
- [Body temperature regulation](https://durgical.com/en/physiology/body-temperature-regulation/)
- [Rh incompatibility](https://durgical.com/en/physiology/rh-incompatibility/)
- [The red cell life cycle and the spleen](https://durgical.com/en/physiology/the-red-cell-life-cycle-and-the-spleen/)

## Related conditions

- [Neonatal jaundice](https://durgical.com/en/conditions/neonatal-jaundice/)
- [Vitamin K deficiency bleeding of the newborn](https://durgical.com/en/conditions/vitamin-k-deficiency-bleeding-of-the-newborn/)
- [Hypothyroidism](https://durgical.com/en/conditions/hypothyroidism/)

## In Biochemistry

- [Newborn screening](https://durgical.com/en/biochemistry/topics/newborn-screening/)
- [Hemoglobin variants (HbA, HbA2, HbF, HbS, HbC)](https://durgical.com/en/biochemistry/topics/hemoglobin-variants/)
- [Bilirubin](https://durgical.com/en/biochemistry/molecules/bilirubin/)
- [UGT1A1 (bilirubin UDP-glucuronosyltransferase)](https://durgical.com/en/biochemistry/enzymes/ugt1a1/)
- [Vitamin K (phylloquinone)](https://durgical.com/en/biochemistry/molecules/vitamin-k/)
- [Heme breakdown and bilirubin](https://durgical.com/en/biochemistry/pathways/heme-breakdown-and-bilirubin/)

[← All of physiology](https://durgical.com/en/physiology/)

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: [28.5 Adjustments of the Infant at Birth and Postnatal Stages](https://openstax.org/books/anatomy-and-physiology-2e/pages/28-5-adjustments-of-the-infant-at-birth-and-postnatal-stages/) (OpenStax (Rice University), 2022); [28.3 Fetal Development](https://openstax.org/books/anatomy-and-physiology-2e/pages/28-3-fetal-development) (OpenStax (Rice University), 2022); [Newborn jaundice: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/001559.htm) (MedlinePlus, US National Library of Medicine, 2026); [Vitamin K deficiency bleeding of the newborn: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/007320.htm) (MedlinePlus, US National Library of Medicine, 2026); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 84). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/newborn-physiology-and-fetal-maturation/