How a baby's weight changes as they grow

A baby's weight is a good indicator of their overall health and development, and it can reassure you that they're feeding well.

So, understandably, parents want to keep an eye on their little one to ensure that they're happy and growing healthily.

All babies are different - just like adults - and their weight can be influenced by several factors. These can include their parents' height and weight, their health when born, and their mother's diet during pregnancy.

To help you understand how your baby's weight may change as they grow, we're going to run through how a baby's weight and height are monitored, and how much weight an average baby gains.

How much weight do babies gain per week?

In their first few days following birth, it's normal for babies to lose some weight - up to 10%. This is because they're born with extra fluid which is then lost. Most little ones will be back to or above their birth weight by two weeks old.  

Although weight changes in babies aren't consistent per week, as a rough guide, after returning to their birth weight, babies generally gain around 175g to 225g (6oz to 8oz) each week. 

Babies gain weight fastest in the first three months and the rate slows steadily after that. As a rough guide, expect around 175g to 225g (6oz to 8oz) a week in the first three months, roughly 100g to 150g a week from three to six months, and around 70g to 90g a week from six to 12 months.

By around six months, most babies weigh double what they did at birth. By 12 months, most have tripled it. 

How do I know my baby is getting enough milk?

Weight is only one signal, and you’ll usually only see it every few weeks. Nappies give you helpful indications every single day, which is why they’re the most practical thing to watch between weigh-ins.

In the first few days, the count builds as your milk comes in:

• Day 1: at least 1 wet nappy
• Day 2: at least 2 wet nappies
• Day 3: at least 3 wet nappies
• Day 4: at least 4 wet nappies
• Day 5 onwards: at least 6 heavy wet nappies every 24 hours

A heavy wet nappy feels like three tablespoons of water poured onto a dry one. Disposables make this hard to judge, so if you’re not sure, weigh one against a dry one. Urine should be pale and almost odourless. Dark, strong-smelling urine, or brick-red or orange staining in the nappy after day three, needs checking by your community midwife or GP the same day.

How stools change from birth

Within 24-28 hours your baby will have the first bowel movement, and will often look thick, sticky and black or dark green. This tar-like substance is called meconium and is a result of amniotic fluid, mucus and cells that your baby swallowed whilst in the womb.

From here, between days 4 and 5, your baby’s stool will transition from meconium to a looser, greenish-brown or yellowish colour.

From around day five to six weeks, expect at least two yellow stools a day, each at least the size of a £2 coin. Before that, you’ll see black meconium in the first two days, then a greenish-brown changing stool around days three and four. After six weeks, breastfed babies often go much less frequently, which is normal as long as they’re otherwise well and gaining.

How is my baby's weight gain monitored?

Once they're born, your baby's growth (including their weight, length, and head circumference) will be monitored by their health visitor or doctor and plotted on a growth chart (also known as a centile chart) at regular intervals. Medical professionals use different newborn weight gain charts for baby boys and girls because their growth patterns are slightly different.

During the first two weeks after birth, they'll be monitored to make sure they return to their birth weight. After those early weeks, a baby is usually weighed...

• once a month until they're six months old
• once every two months between the age of six months and one year
• once every three months after their first birthday.

If your doctor says your baby is healthy and growing well, there's usually no reason to worry about their weight. But if there's a possible health issue to be investigated, your baby's growth may be monitored more regularly.

What is the average baby's weight?

This chart is from the WHO (World Health Organisation) and is just a rough guide to the average 50th centile baby weights in their first year. Babies who are born prematurely before 37 weeks are monitored using a slightly different weight chart.

When should I worry about my baby's weight gain?

All babies are different and baby weights under or above the figures stated in the chart above aren't necessarily something to worry about. It's normal for babies to go through a period of not gaining weight or gaining more weight than usual. This can be due to spurts in their growth, a sickness, issues with them adjusting to solid foods, or simply because they burn more calories once they start to become more active.

As with all aspects of parenting, it's good to trust your instincts! So, if you've got any worries about your baby's growth at any stage, don't be afraid to reach out to your doctor or health visitor.

Do breastfed and formula-fed babies gain weight differently?

Yes. Breastfed babies typically gain faster than formula-fed babies in the first two to three months, then more slowly from around three to twelve months. Formula-fed babies tend to be heavier by the end of the first year.

Both patterns are completely normal. What a health visitor looks at when looking at weight gain is the shape of the curve over time, not a single point or the feeding method you’ve chosen to use. A baby who is following their own centile line is growing well, whether that line is the 9th or the 91st.

Why isn’t my breastfed baby gaining weight?

Slow weight gain in a breastfed baby is almost always about how much milk is being transferred, not about whether you’re capable of producing it. Here are some common causes of a breastfed baby gaining weight slowly:

A shallow latch

If your baby isn’t taking enough breast tissue into their mouth, they can feed for a long time and still transfer very little. Signs include sore or misshapen nipples after feeds, clicking sounds, very long feeds, and a baby who never seems satisfied. A latch assessment from someone trained usually resolves it.

Tongue-tie

A restrictive tongue-tie can prevent effective milk transfer. It can also cause the same signs as a shallow latch, which is why it often gets missed. It needs assessment by a trained practitioner, and division is a quick procedure when it’s indicated.

Scheduled or restricted feeding

Breast milk production works on supply and demand. Feeding to a routine, stretching gaps, or capping feed length reduces both the milk your baby gets and the signal to make more. Responsive feeding (feeding when your baby shows hunger cues, as often as they want) is what supports supply.

Dummy use masking hunger cues

In the early weeks before feeding is established. Once breastfeeding is going well, this is much less of a concern.

A sleepy baby

This is particularly for babies that are jaundiced, premature or small. A baby who doesn’t wake to demand feeds often may need waking for them.

Low supply

Low supply is less common than people think but is real. It can relate to retained placenta, significant blood loss at birth, thyroid conditions, PCOS, previous breast surgery, or certain medications, and diet. It’s worth investigating properly rather than guessing. Learn how to increase your breast milk supply with our guide.

How can I help a baby who is gaining slowly?

Slow weight gain can have a medical cause, and the approaches below work best alongside proper guidance from your midwife, health visitor or GP.

If you’re breastfeeding:

• Feed more often rather than for longer. More frequent feeds transfer more milk and signal your body to make more. Aim to feed responsively, including at night, when prolactin levels are at their highest.
• Get the latch checked. This is the single highest-value thing you can do, and it needs eyes on a feed rather than a description over the phone.
• Try breast compressions. While your baby is feeding, cup your breast and squeeze gently and steadily when their sucking slows. This helps keep milk flowing and keeps a sleepy baby actively feeding. Release when they stop swallowing, and repeat.
• Offer both breasts at each feed, and offer the first side again once the second is done.
• Try switch nursing: move your baby to the other side each time their swallowing slows, several times per feed. It keeps them actively drinking rather than comfort sucking.
• Keep your baby close. Skin-to-skin contact supports both supply and feeding frequency. 
• Consider expressing with a breast pump after feeds if your specialist recommends it, which protects supply and can provide extra milk to offer.

If you’re formula feeding:

• Offer feeds responsively, following your baby’s hunger cues rather than the clock.
• Check the teat flow. A flow that’s too slow means a baby tires before finishing; too fast and they may take in air and stop early.
• Check how the feed is going: a baby who falls asleep after 20ml, takes an hour over a bottle, or regularly brings the feed back up is worth mentioning.
• Keep a short record of volumes and times to take to your appointment if you’re concerned.

Never add extra formula powder to a bottle, use less water than the instructions state, or add anything to formula to increase its calorie content. Concentrated feeds put strain on a baby’s kidneys and can cause serious dehydration. Always follow the manufacturer’s instructions exactly.

High-calorie formulas and fortifiers do exist, and they’re prescribed and monitored by a paediatric dietitian or GP for specific clinical reasons.

If your baby has started solids, keep offering milk feeds first for now. Milk is more calorie-dense than most first foods, and filling up on vegetables before a feed can reduce overall intake rather than increase it.

Weight gain when you’re exclusively pumping

Exclusively pumping gives you something breastfeeding parents don’t have: an exact number for every feed.

What is a typical intake?

From around one month to six months, a baby’s daily milk intake stays relatively stable at roughly 750ml to 1,035ml in 24 hours, with an average near 800ml. It doesn’t climb the way formula feeding guides sometimes imply, because breast milk composition adapts as your baby grows. A rough starting point is 150ml per kilogram of body weight per day for a baby under six months, but go by your baby rather than the maths.

Weight gain expectations are the same as for any breastfed baby: fastest in the first three months, slowing after that. Your baby is receiving breast milk, so the breastfed growth pattern applies.

What to watch alongside the numbers:

• Nappy output, using the same counts as above. It’s the better day-to-day signal.
• Whether your baby finishes bottles, and whether they seem content afterwards.
• Your own output trend over a week rather than session by session, since individual pumps vary enormously.

Speak to your health visitor if your baby’s intake drops noticeably, if wet nappies reduce, or if weight gain slows. And if your own supply is falling, an IBCLC can help with a pumping plan, which is a specialist area in its own right.