Why it happens

Newborns are born carrying extra fluid, and they lose it. The AAP explains: "When your baby was born, their weight included excess body fluid, which they lost during their first few days." Raising Children Network says the same: "Breastfed and bottle-fed newborns typically lose weight during the first 5 days after birth. This happens as they lose excess fluid."

This is expected, universal and not a sign that feeding is failing. The NHS states it directly: "It's normal for babies to lose some weight in the first few days after birth."

How much, and by when

The three figures worth knowing:

How much loss is acceptable. Raising Children Network: "This weight loss shouldn't be more than 10% of their birth weight." For a baby born at 3.5 kg (7 lb 11 oz), that is a floor of about 3.15 kg (6 lb 15 oz).
When gaining starts. The AAP: "All newborns lose weight during the first 7 days. Beginning once they are about 5 days old, they should start gaining weight steadily."
When birthweight is back. The AAP: "by about 2 weeks of age, they should be back at their original birth weight. Almost all babies will have regained their birth weight by 3 weeks of age." The NHS agrees: "Most babies are at, or above, their birthweight by 3 weeks." Raising Children Network says most newborns weigh more than they did at birth after 1 to 2 weeks.

NICE's postnatal care guideline (NG194) sets the monitoring schedule: "Measure weight and head circumference of babies in the first week and around 8 weeks, and at other times only if there are concerns. Plot the results on the growth chart."

The NHS describes what happens if the numbers do not look right: "A midwife or health visitor will support you if your baby loses a large amount of weight or does not regain their birthweight by 3 weeks. They'll talk to you about how feeding is going, possibly ask to observe a feed if you're breastfeeding, and look at your baby's health in general." That is the correct response and it is not a judgement on you.

What normal gain looks like afterwards

The AAP gives concrete figures for the first month:

Weight: "The average newborn gains weight at a rate of almost an ounce (28 grams) per day."
Length: babies "tend to grow between 1½ and 2 inches (3.8 to 5 cm) during this month."
Head circumference: from about 35 cm (13¾ inches) at birth to about 38 cm (15 inches) by 1 month. The AAP notes paediatricians pay close attention to head growth because it reflects brain growth.
Growth spurts typically occur at around 7 to 10 days old, and again between 3 and 6 weeks — which is when feeds suddenly become relentless again.

The NHS notes weight gain is fastest in the first 6 months and slows as babies become toddlers, and that illness can slow gain for a while: "It will usually return to normal within 2 to 3 weeks."

How often your baby should be weighed

Less often than most parents expect, and the NHS is specific about it. After the first 2 weeks, a baby should be weighed:

No more than once a month up to 6 months of age.
No more than once every 2 months from 6 to 12 months.
No more than once every 3 months over the age of 1.

More frequent weighing happens only if you ask for it or if there are concerns. Weighing a baby every few days produces noise, not information.

Reading the centile chart

Growth is recorded on centile charts in your baby's personal child health record. The NHS explains what the lines mean: "These show the average weight and height gain for babies of different ages. Your baby's weight and height may not follow a centile line exactly. Their measurements may go up or down by 1 centile line, but it's less common for them to cross 2 centile lines. If this happens, talk to a health visitor."

Two things worth internalising. First, the charts show the pattern healthy children usually follow whether breastfed, formula fed or both. Second, boys and girls have different charts, and "it's normal for your baby to be on different centiles for weight and length." Examples of the UK-WHO growth charts are published by the Royal College of Paediatrics and Child Health.

The important part is the trajectory, not the number. A baby tracking steadily along the 9th centile is doing better than a baby dropping from the 75th to the 25th.

The signs that matter more than the scales

NICE's feeding assessment asks about the things you can observe at home: whether the baby shows readiness to feed, rhythmic sucking and audible swallowing, whether they seem content after a feed, whether they wake for feeds, and the number of wet and dirty nappies.

Concrete benchmarks from the NHS and the AAP:

Poo: "Because breastmilk is a natural laxative, a baby aged between 4 days and 6 weeks who is breastfeeding well should pass at least 2 yellow poos a day." If your baby has not pooed in the past 24 to 48 hours, the NHS says to speak to your midwife or health visitor — it may mean they are not getting enough milk.
Wee: the AAP says that within 3 to 5 days, breastfed and bottle-fed babies alike "will wet about 8 diapers every 24 hours."
Poo colour progression: black tarry meconium first, changing by day 3 to a lighter, loose, greenish poo, then mustard-yellow and seedy in breastfed babies or pasty brown in formula-fed ones. The AAP notes this transition "suggests your baby is getting plenty to eat."

Raising Children Network adds one more: some newborns need waking for feeds — particularly "newborns who have lost a lot of weight, are very small or are jaundiced."

When to call

Contact your midwife, health visitor or GP as soon as possible if your baby:

Has lost more than about 10% of their birthweight, or has not regained birthweight by 3 weeks.
Has fewer than 6 to 8 wet nappies a day (Raising Children Network).
Is taking half the normal volume or number of feeds in a 24-hour period, or vomits more than half of 3 feeds in a row (Raising Children Network).
Has not pooed in 24 to 48 hours in the first 6 weeks, if breastfed (NHS).
Seems irritable, lethargic, very tired all the time, or is hard to wake for feeds.
Has pale or yellow skin — jaundice and poor feeding often travel together, and each makes the other worse.

Seek urgent care if your baby is unrousable, has a temperature of 38C (100.4F) or above or below 36C (96.8F), is breathing fast or grunting, or looks pale, mottled or blue. NICE lists all of these among its red flags for serious illness in young babies, and a baby who has stopped feeding is one of the situations where those signs appear.

One last note on NICE's definition, because the term gets used loosely: low birth weight means "a birth weight of less than 2,500 grams regardless of gestational age" — about 5 lb 8 oz. It is a separate concept from the weight a baby loses after birth, and it carries different follow-up.

Newborn Weight Loss and Regain in the First Week