[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/areas/repro/) › Fetal metabolism and the newborn's nutritional needs

# Fetal metabolism and the newborn's nutritional needs

- Concept
- Exam priority: medium

The fetus runs on maternal glucose and stores fat, glycogen, iron and calcium late in pregnancy. Newborns are born short of vitamin K and vitamin D.

Updated Nov 13, 2026

Also called: Third-trimester accretion, Neonatal glucose transition

## Key points

- Glucose is the main fetal fuel, delivered by facilitated diffusion; fetal glucose follows maternal glucose at a lower level.
- Fetal insulin acts as a growth hormone: maternal hyperglycemia → fetal hyperinsulinemia → macrosomia and neonatal hypoglycemia.
- About 80% of fetal calcium and phosphate, most body fat and much of the iron are gained in the third trimester: preterm infants miss them.
- Vitamin K crosses the placenta poorly, the newborn gut is sterile and breast milk is low in it: hence routine vitamin K at birth.
- Breast milk is low in vitamin D; breastfed infants with little sun exposure need supplementation to avoid rickets.
- Term iron stores last about 4–6 months; preterm stores run out earlier.

## What it is

The fetus gets a continuous supply of glucose, amino acids and minerals from the placenta and does little gluconeogenesis of its own. Most of its fat, glycogen, calcium, phosphate and iron is laid down in the third trimester, so preterm babies are born with small reserves. At birth the supply stops abruptly: the newborn must make its own glucose, regulate calcium and rely on milk, which is low in vitamin K, vitamin D and iron.

## Key facts

- Term body fat is about 12–16% of body weight, mostly stored in the last 10 weeks; brown fat supports heat production without shivering.
- Small-for-gestational-age, preterm and stressed newborns have small glycogen and fat stores and the highest hypoglycemia risk.
- Vitamin C: the newborn starts with stores from the mother and gets it from breast milk; unfortified cow's milk is a poor source.
- Energy needs per kg are high (≈ 100–120 kcal/kg/day at term) because of growth and a large surface area for heat loss.
- Delayed cord clamping moves extra placental blood to the baby and adds to iron stores.

## How it works, step by step

- Placental supply of glucose keeps fetal insulin moderate and promotes glycogen and fat storage late in gestation.
- Cord clamping stops the glucose supply; plasma glucose falls to a physiological low point in the first 1–2 hours.
- Catecholamines and glucagon rise and insulin falls: liver glycogen breaks down, then gluconeogenesis and ketogenesis start.
- Liver glycogen is largely used up within the first day, so feeds and gluconeogenesis must take over.
- The placental calcium pump also stops: total calcium falls to a low point at 24–48 h, and the parathyroid glands respond by raising PTH.
- Red cells made in utero break down after birth; the iron is recycled into stores that support growth until about 4–6 months.

## Newborn nutrients at risk and why

| Nutrient | Why the newborn is short | Result if lacking |
| --- | --- | --- |
| Glucose | Placental supply stops; glycogen lasts hours | Neonatal hypoglycemia (jitters, poor feeding, seizures) |
| Calcium and phosphate | Placental transfer stops; preterms miss third-trimester accretion | Early neonatal hypocalcemia; metabolic bone disease of prematurity |
| [Vitamin D](https://durgical.com/en/conditions/rickets-and-osteomalacia/) | Low in breast milk; little sun | Rickets |
| [Vitamin K](https://durgical.com/en/conditions/vitamin-k-deficiency-bleeding-of-the-newborn/) | Poor placental transfer, sterile gut, low in milk | Vitamin K deficiency bleeding |
| [Iron](https://durgical.com/en/conditions/iron-deficiency-anaemia/) | Stores from the third trimester; low in milk | Iron deficiency anemia, earlier in preterms |

## Clinical notes

- Infants of diabetic mothers: large, with transient hyperinsulinemia and hypoglycemia after birth, sometimes hypocalcemia too.
- Metabolic bone disease of prematurity: low phosphate, high alkaline phosphatase and poor mineralization when intake cannot match fetal accretion.
- Vitamin K deficiency bleeding can appear early (first day), classically at 2–7 days, or late (weeks), when it may involve the brain.

## Memory aid

'K and D are not in the milk, iron lasts half a year, and glucose lasts hours.'

## Test yourself

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

## Why does the infant of a mother with poorly controlled diabetes become hypoglycemic after birth?

Maternal hyperglycemia drove fetal β-cell hyperplasia and high insulin; when the glucose supply stops at birth, insulin stays high and glucose falls.

## Why are preterm infants at risk of metabolic bone disease?

About 80% of fetal calcium and phosphate is gained in the third trimester; a preterm birth misses most of it, and milk cannot easily supply the same rate.

## Which three factors make newborns vitamin K deficient?

Poor placental transfer, a gut without vitamin K–making bacteria and low vitamin K in breast milk.

## Until about what age do a term baby's iron stores last?

About 4–6 months; stores come from the third trimester and red cell breakdown after birth. Preterm infants run out sooner.

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## Connections

## Related topics 3

- [Newborn physiology and fetal maturation](https://durgical.com/en/physiology/newborn-physiology-and-fetal-maturation/)
- [Lactation](https://durgical.com/en/physiology/lactation/)
- [Body iron balance](https://durgical.com/en/physiology/body-iron-balance/)

## Related conditions 5

- [Neonatal hypoglycaemia (including infant of a diabetic mother)](https://durgical.com/en/conditions/neonatal-hypoglycaemia/)
- [Vitamin K deficiency bleeding of the newborn](https://durgical.com/en/conditions/vitamin-k-deficiency-bleeding-of-the-newborn/)
- [Iron deficiency anaemia](https://durgical.com/en/conditions/iron-deficiency-anaemia/)
- [Rickets and osteomalacia](https://durgical.com/en/conditions/rickets-and-osteomalacia/)
- [Prematurity and its complications](https://durgical.com/en/conditions/prematurity-and-its-complications/)

## In Biochemistry 2

- [Vitamin K (phylloquinone)](https://durgical.com/en/biochemistry/molecules/vitamin-k/)
- [Vitamin D (calcitriol)](https://durgical.com/en/biochemistry/molecules/vitamin-d/)

[← 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: [Low blood sugar - newborns: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/007306.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); [Rickets: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000344.htm) (MedlinePlus, US National Library of Medicine, 2026); [Iron Deficiency and Microcytic Hypochromic Anemia](https://www.ncbi.nlm.nih.gov/books/NBK560876/) (StatPearls (NCBI Bookshelf), 2023).

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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/fetal-metabolism-and-the-newborn-s-nutritional-needs/