Chubby cheeks… thunder thighs… squishable, squeezable folds of baby fat. Think of a cuddly, well-fed infant, and these images likely spring to mind. After all, it’s pretty well ingrained in our collective psyche that a chubby baby is a healthy baby.

But what about those babies who are on the skinnier side? When your little bundle doesn’t look like a mini sumo wrestler, grandparents, friends, and even strangers may come out of the woodwork at surprising speed to comment on your child’s size.

“Are you feeding him?”

“Maybe you should supplement with formula!”

“When are you starting her on solids?”

These are just some of the (often unsolicited) remarks parents of thin babies may hear.

It’s easy to fall prey to fears about your baby’s weight when they don’t look like the cover model of Chubby Babies Monthly — but just like with adults, there’s a wide range of what’s healthy for an infant’s size and shape.

Of course, situations do exist where babies need to gain more weight, but it’s possible your little one is just fine at a more petite size. Wondering how thin is too thin? Here’s what you need to know.

When your pediatrician tells you where your baby falls “on the chart,” they’re likely referring to the World Health Organization’s (WHO) growth charts, which the Center for Disease Control and Prevention (CDC) recommends for children from birth to age 2. (The CDC has its own growth charts for ages 2 and up.)

These charts are based on years of high-quality research and use breastfed infants as their norm. Separate charts exist for girls and boys. Your doctor may use the WHO’s weight-for-age or weight-for-length charts — or both.

The WHO charts plot your baby’s data points of length or weight on one axis and their age on the other. Wherever the two meet on the graph determines your baby’s percentile for their age.

Understanding growth percentiles

To understand what the numbers mean, consider this example: A child in the 25th percentile for weight has a higher weight than 25 percent of children their age.

And don’t forget that, on a growth chart where the 50th percentile is considered average, 49 babies out of 100 will be “lower than average.” That’s a lot of babies!

In general, an infant born at term is considered underweight when their weight-for-age measurement is in the 5th percentile or less. (This is not necessarily the case if your baby was born premature or with certain health conditions.)

If you’d rather not wait until your next doctor’s appointment to know where your little one is on the curve, you can calculate their percentages by plotting their length and weight on the WHO’s online charts.

It’s also worth noting that, while adults often assess weight using the body mass index (BMI), the CDC doesn’t recommend this for infants.

Related: What’s the average baby weight by month?

So you’ve done the whole chart thing, talked with your baby’s pediatrician, and your baby isn’t underweight. Phew. So what’s with the lack of baby fat rolls?

Genetics

Sometimes, in our desire to have a baby at a certain point on the growth chart, we can forget just how much our genes may play a part in children’s size.

So ask yourself: How large am I? How large is the baby’s other parent? If you and/or your baby’s other parent are smaller people, it’s only logical that your child may be, too.

However, it’s also true that the genetics of size may not show up until after infancy. In the first year or two, a baby’s weight may be more related to their birth weight.

Low birth weight

If your baby was born at a low birth weight due to early or premature delivery, or as a result of being a multiple, they may continue to be small for the first several months of life, or longer.

Remember, too, that babies born at low, normal, or high weight can fluctuate in their progress. A moderate slip on the growth curve can be a normal part of the two-steps-forward-one-step-back dance of baby growth — but talk to your doctor if you notice this happening. There are instances where a regression on the growth curve is an indicator of a problem.

Breastfed vs. bottle-fed

It may sound like a stereotype, but breastfed babies and bottle-fed babies often do have differences in weight gain in the first year of life. A 2012 study found that the more infants were breastfed, the less weight they had gained at 3, 5, 7, and 12 months. Conversely, the more bottle feedings the babies received, the higher their weight.

This doesn’t mean that a breastfed baby needs to be bottle-fed for healthy growth — or that most formula-fed babies are overweight! Observing other important indicators of well-being in your baby can give you peace of mind about the number on the scale.

When your baby doesn’t have rolls, another important question may be whether they can do rolls. In other words, meeting milestones is often a better indicator of your child’s well-being than how squishy they are.

Your pediatrician can inform you about when to watch for age-based milestones like smiling, holding up their head, rolling over, and bearing weight on their legs. These all help show baby is progressing just fine.

Other signs that may reassure you that your baby is healthy but lean include regular wet diapers (at least four or five per day), consistent poopy diapers, and an alert, happy temperament.

Related: How often do breastfed and formula-fed babies poop?

On the other hand, if your little one’s milestones seem to be delayed — or if they’re not meeting them at all — it’s time to check in with your pediatrician.

Likewise, if baby’s slow growth is accompanied by any of the following, seek medical attention:

  • lethargy
  • not feeding well at the bottle or breast
  • not producing wet or soiled diapers

When a baby doesn’t gain enough weight, they may be termed as having failure to thrive. According to the American Academy of Family Physicians, this diagnosis is usually given when your child’s weight falls below the 5th percentile on standard growth charts.

Failure to thrive can sound scary, but it’s not necessarily a permanent sentence of doom and gloom. In many cases, it’s a temporary condition that has to do with poor feeding at the breast or bottle. Oftentimes, this will resolve when feeding interventions get baby’s weight back on track.

In some cases, failure to thrive is the result of a genetic or health condition. Babies with Down syndrome, heart conditions, cystic fibrosis, cerebral palsy, and other underlying disorders can all have problems with growth. Digestive conditions like acid reflux or celiac disease can also keep your little one from eating well, resulting in poor growth.

Individual growth charts have been developed for children with a variety of special needs, such as Down syndrome, Prader-Willi syndrome, and Marfan syndrome. If your child has a health condition, your pediatrician may choose to use one of these specialized charts to monitor their growth more accurately.

Still worried about baby’s weight? It’s always smart to consult your pediatrician about any concerns regarding your child’s growth.

A trusted doctor can walk you through warning signs that your child is failing to thrive, as well as make a physical assessment in person. They can also take consistent measurements at well-child visits to track how your baby’s growth progresses over time.

If nothing else, seeing your doctor can give you much-needed peace of mind. Even when others may claim you should take steps to “fatten up” baby, your pediatrician is the authority who can make this call.

Every situation is different when it comes to helping your little one put on weight. If you’ve established that underweight is genuinely a problem for your child, talk to your doctor about how to best address it.

If you’re breastfeeding, your pediatrician may advise you to nurse your baby more often or take other measures to boost your supply. They may also instruct you to supplement with formula or begin (or increase) solid foods. Parents of formula-fed babies may also be instructed to add more feedings or finger foods.

For babies who have started solid foods but still aren’t eating enough, weight-promoting options include offering more variety, choosing higher-calorie, more nutrient-dense foods, and working to make mealtimes an appetizing, pleasant experience.

Long, short, thin, or chubby — babies come in all shapes and sizes. While society may send a message that your lil’ nugget has to have rolls of baby fat to be healthy, this isn’t really true.

As long as they’re meeting developmental milestones, are alert and active, and are feeding well, you likely don’t need to worry. Consistent growth — not exponential growth — is the key to baby’s well-being.

Remember, too, that your pediatrician — not your neighbor or your aunt Sheila — is the best expert to determine whether your baby needs to gain more weight. Even if your little one does need to bulk up, there are plenty of tools and resources for getting them back on weight-gaining track.