Baby weight by age: how much should my baby weigh?

Most babies gain roughly 150 to 200 g (about 5 to 7 oz) a week in the first three months, and less as they get older. Birth weight usually doubles by around five months and triples by about a year. Healthy babies vary widely, so the trend over several weeks matters far more than any single number.

How much should a baby weigh? The short answer

There is no single correct weight for a baby of a given age. There is a wide healthy range, and your baby's own line across time says far more than one reading on one afternoon.

That is what a growth chart is for. It does not set a target. It shows how a large group of healthy babies were spread out at each age, so your doctor can see where your baby sits and, more importantly, whether your baby keeps growing along a steady path of their own.

One thing worth knowing: for babies under two, the United States, the UK, Canada and Australia all use charts built on the same data, the World Health Organization Child Growth Standards. Whether the numbers end up in a red book, a personal health record, a well child visit summary or a clinic screen, the underlying curves are the same. That is why a friend overseas can compare notes with you and find the chart familiar.

What a chart cannot do is tell you whether your baby is well. Only a clinician who can weigh your baby, look at them, watch a feed and take your history can say that.

Average baby weight by age

The tables below show weight by age for boys and girls born at term, from birth through the second year. Read the middle column as the centre of the range, not the goal.

On a phone, swipe the table sideways to see every column.

Figures come from the WHO Child Growth Standards and apply to babies born at term; babies born preterm are usually plotted using corrected age or a chart supplied by their own doctor.

Reading it is simpler than it looks. If your baby's weight sits near the 3rd percentile line, roughly 3 babies in 100 of the same age and sex weigh less. Near the 97th, roughly 3 in 100 weigh more. Both ends of the chart are full of thriving babies. Somebody has to be at the small end, and small is not the same as unwell.

Labelling differs a little by country. British charts talk about centiles and print extra lines at the outer edges; North American and Australian charts usually say percentiles. Same idea, different wording. Boys and girls are read on separate charts because they grow at slightly different average rates from the start.

The first two weeks: the newborn weight dip

Nearly every newborn loses weight in the first days. It is expected, and it still catches parents off guard, because very few of us are warned about it on the way out the door.

Babies are born carrying extra fluid, they pass meconium, and a full milk supply takes a few days to arrive. A loss of up to about 7 to 10 percent of birth weight over the first three to four days is generally treated as within the usual range. Most babies then stop losing, start gaining, and are back to their birth weight by around two weeks.

Because of that dip, weight in the first fortnight is normally compared with birth weight rather than read off a chart at all. Your midwife, health visitor, nurse or doctor will usually weigh your baby a few times in this period and watch the direction of travel rather than any one figure.

If the loss goes beyond about 10 percent, or your baby is not back to birth weight by two to three weeks, that is a prompt for a proper feeding assessment: someone watching a whole feed, checking latch and positioning, checking output. It is not a cue to change what or how you feed on your own. Any change to feeding, including top ups, should be talked through with your doctor, midwife, health visitor or child health nurse first.

How fast babies actually gain weight

Parents ask about gain as often as they ask about totals, and it is the more useful question. Typical average weekly gain, once the newborn dip is over, looks roughly like this:

• Birth to 3 months: around 150 to 200 g a week (about 5 to 7 oz).
• 3 to 6 months: around 100 to 150 g a week (about 3.5 to 5 oz).
• 6 to 12 months: around 70 to 90 g a week (about 2.5 to 3 oz).
• Second year: slower again. Toddlers are constantly moving, appetite swings from week to week, and the curve flattens noticeably compared with the first year.

Alongside that pattern, most babies roughly double their birth weight by about five months and roughly triple it by about a year. These are averages with a great deal of healthy variation either side. A baby born on the larger side often gains proportionally less; a smaller baby may gain faster for a stretch and then settle.

Gain is also not smooth. Babies grow in bursts, and a quiet week followed by a jump is completely ordinary. This is exactly why one weighing tells you so little, and four weighings over six weeks tell you a great deal. If the percentile line on your chart is the part that worries you, our guide to what a baby's growth percentile actually means explains how to read that number and when a change in it matters.

Breastfed and formula fed babies follow slightly different curves

In the first month or two, gain is broadly similar. After that, breastfed babies tend to gain a little more slowly than formula fed babies, and the difference is most visible somewhere between about three and twelve months.

This is normal and it is not a fault in either direction. The WHO standards were built on healthy breastfed babies, so a formula fed baby may track a little above the middle of the chart in the second half of the first year, while a breastfed baby may sit slightly below the line they started on. On its own, that is not a reason to change anything.

What matters is that the drift is gentle and the baby is well: alert, feeding, plenty of wet nappies, hitting their milestones. If a breastfed baby genuinely is not gaining, the answer is a feeding review with a professional who can watch a feed, not a tin of formula bought on the strength of a chart. Speak to your doctor, midwife or health visitor before adding formula, topping up, changing how milk is prepared, or altering when solids start.

What can make the number look wrong

A surprising share of "my baby has lost weight" panics come down to how the weighing was done. The usual culprits:

• A different scale. Two scales can disagree by more than an entire week's gain. Clinic scales are calibrated and checked against a standard. Home scales and bathroom scales usually are not.
• Clothes and nappies. A vest, a sleepsuit and a full nappy or diaper easily add a few hundred grams. Weighed dressed one week and undressed the next, your baby "lost" weight without anything having happened.
• Feed timing. Before a feed and after a feed are two different numbers. So are before and after a substantial nappy.
• Weighing too often. Daily weighing mostly measures milk going in and nappies coming out. It manufactures worry rather than information.
• The double weigh trick. Standing on bathroom scales holding your baby and subtracting your own weight sounds sensible, but the margin of error is often larger than the gain you are trying to detect.

So compare like with like: same scale, same conditions, same rough point in the feeding day. Then judge the trend across several weighings instead of reacting to one. If a home number frightens you, the sensible next step is a weighing at your clinic, not a second attempt on the bathroom floor.

When a weight really does need checking

Most weight questions are answered by watching the trend over time. Some are not, and these are the ones that belong with a person rather than a chart.

Call your doctor, health visitor or child health nurse if:

• Your baby has no wet nappies, or far fewer than usual for their age.
• Your baby has not returned to birth weight by about two to three weeks.
• Your baby loses weight after the first two weeks, or the weight stays flat across several weighings.
• The weight crosses downwards through two or more major percentile lines on the chart.
• Your baby is unusually sleepy, hard to wake or floppy, or is feeding poorly, feeding only very briefly, or refusing feeds.
• Feeding has become painful, endless or distressing for either of you.
• You simply feel that something is wrong. That instinct is a legitimate reason to be seen, and clinicians would far rather check.

Seek urgent, same day care if your baby is difficult to rouse, will not feed at all, is breathing fast, or looks unwell to you.

Worth naming honestly: sometimes a low weighing is just a low weighing, and sometimes it is the first sign of something a doctor can help with, such as a feeding difficulty, tongue tie, reflux, an infection, or a condition affecting how food is absorbed. A website cannot tell you which one you are looking at. That is not a hedge, it is simply true. What a website can do is help you arrive at the appointment with good information instead of a half remembered number.

One last thing, for the parent who found this page after a hard appointment. A single number started this worry, and a single number is almost never the whole story. Take your dates and weighings to your next visit, ask what the pattern looks like over time, and ask what specifically would be watched next and when. A clear answer from someone who has actually seen your baby is worth more than any table on the internet, including this one.