Why do newborns lose weight after birth?

It is expected and normal. Almost all newborns lose weight in the first days, and understanding why prevents a great deal of unnecessary anxiety.

The usual figures: a loss of up to about 7–10% of birth weight over the first three to five days, with birth weight typically regained by 10 to 14 days.

Why it happens:

Fluid loss, which is most of it. Babies are born with substantial extra fluid, and they pass it in the first days through urine and stool. Babies born after intravenous fluids were given to the mother during labour — common with epidurals and inductions — carry more fluid and consequently show larger apparent losses. Some research suggests birth weight in these cases is artificially inflated, so the "loss" partly reflects a measurement artefact.

Passing meconium, the dark sticky first stool accumulated in utero, which has real mass.

Low intake initially. Colostrum — the first milk — is produced in very small volumes, measured in teaspoons rather than millilitres of mature milk. This is not a deficiency: it is highly concentrated, rich in antibodies, and matched to a newborn stomach that is genuinely tiny. Mature milk typically comes in around days two to four.

What is monitored instead of weight alone: the number of wet and dirty nappies, which is the most useful day-to-day indicator; feeding frequency and effectiveness; alertness and tone; and jaundice.

When it needs attention. Loss exceeding roughly 10%, failure to regain birth weight by around two weeks, very few wet nappies, a sleepy baby difficult to rouse for feeds, or signs of dehydration all warrant prompt review by a midwife, health visitor or GP. Excessive weight loss can also worsen jaundice.

It is not evidence that breastfeeding is failing. Early weight loss is physiological, and support with positioning and attachment resolves most genuine feeding difficulties.