A newborn weighed on the third or fourth day usually weighs less than at birth. The loss is expected, has identifiable causes, and is monitored rather than treated as a surprise.
Babies are born carrying extra fluid
A foetus develops in fluid and is born with a higher proportion of body water than it will carry later. Some of that surplus is lost in the first days.
Fluids given to the mother during labour can add to this, since they cross to the baby and inflate the birth weight recorded moments after delivery.
Where that happens, part of the apparent loss is a correction of an artificially high starting figure rather than a true reduction in tissue.
Milk volume increases over several days
Colostrum is produced in very small volumes and is highly concentrated. It is well matched to a newborn stomach, which is extremely small in the first day.
The transition to larger milk volumes typically occurs over the following days rather than immediately, so intake in that window is deliberately modest.
Weight loss during this period is therefore part of the normal sequence, and the expected pattern is a low point followed by steady regain.
Meconium and other losses count
The gut contains meconium accumulated before birth, and passing it removes measurable weight. It is dark and sticky and is usually cleared over the first days.
Urine output rises as feeding establishes, and fluid is also lost through the skin and breathing, which is more significant in a small body.
None of these is a feeding problem. They are the reason a weight taken on day one and a weight taken on day three are not measuring the same thing.
How the loss is monitored
Services track the loss as a proportion of birth weight, watch for the day the loss stops, and check that birth weight is regained within an expected period.
Thresholds exist above which a feeding assessment is triggered, and those thresholds differ between countries and between guidelines, as does the expected day of regain.
Because scales, clothing and timing all affect the figure, a single unexpected reading is usually repeated before conclusions are drawn.
What is looked at alongside the number
Weight is interpreted with feeding frequency, how the baby latches or takes a bottle, alertness, and the number of wet and dirty nappies over a day.
A baby losing more than expected is assessed for feeding effectiveness rather than simply fed more, since the usual cause is transfer of milk rather than supply alone.
Concerns about a newborn's weight, feeding or alertness belong with a midwife, health visitor or doctor promptly, since the margin in a very small baby is narrow.