Why Do I Have A Skinny Baby?

As a new parent, let’s say you have a skinny baby, and people keep telling you he is underweight. Is that a call for concern? Maybe but that’s only sometimes the case.

The people of the 21st century are obsessed with physical appearance and fitting in, so parents can notice any abnormality in their baby’s growth or weight gain, especially when their baby is becoming too skinny. 

However, not everyone can tell when a baby is overweight by merely looking at them. This becomes a problem because they may think an obsessed child is healthy when he is not.

The same goes for skinny babies. Most people would consider a skinny baby isn’t healthy while he is.

It is essential to know that when you have a skinny baby, it doesn’t necessarily mean he is sick or malnourished. This is because this might be how they were born genetically. 

The body mass index of some babies won’t increase no matter how much food is given to them, and this is because they are genetically slim and have a quick metabolism. However, it doesn’t necessarily mean they are sick or malnourished. 

The only thing he lacks is a heavy coating of body fat.

However, that is also the problem lies. 

So, how can you tell if your child is skinny due to other factors or if being skinny can be due to malnutrition and genetics?

We will discuss this in detail in this article and discuss how you can fix the problem.

Misdiagnosis Of Genetically Skinny Babies

Why Do I Have A Skinny Baby
Credit: Aree Thaisagul/iStockphoto

Compared to the usual weight gain for infants of the same age, the growth of genetically slim babies is frequently misunderstood as poor growth. ‘Failure to thrive’ is a common misdiagnosis for many genetically thin infants.

Even though it is simple to distinguish between genetically lean babies and those underfed, many medical professionals fail to do so (by asking parents pertinent questions about their child’s behavior and wet or soiled diapers) because they assume that an underweight baby is also underfed.

Why Your Baby Is Skinny And Underweight


A baby can be lean due to their genetics. But, sometimes, we forget how much our genes may influence the size of our children.

Therefore, consider this: How big am I? How big is the other parent of the child? It is reasonable for you to have a minor child if you are small and vice versa. This is because your child inherited the body size of one or both of his parents.

Sometimes children are born underweight and be given an intrauterine growth restriction (IUGR) diagnosis. However, he might also have had an average birth weight and gradually lost weight during the first few months.

By medical standards, when our child is genetically thin, it is considered underweight. That is to say, the parents’ genes or family members can determine a baby’s size.

A baby is said to be underweight when his weight is two or more percentile curves below his length in the growth chart.

Here are the signs which can help you know when your genetically lean baby is well-fed:

  • If he wants to be fed frequently. 
  • If he seems satisfied after being fed. You can observe this by his reaction when you make him eat more than he cares for.
  • If he smiles easily when he is interacting with others.
  • If he is energetic and alert whenever he is awake. 
  • If he is wise and curious about his environment.
  • If he sleeps easily.
  • If he wets his diapers five or more times per day.
  • If he has normal bowel movements. However, this can differ depending on what he is fed. For example, if he is formula-fed, this can range from 3 times per day to once in three days. And more or less if he was breastfed.
  • If he achieves all developmental milestones 

It is not possible to fatten a genetically lean baby. This is because he will only consume as much food as his body requires.

Additionally, the homeostatic mechanisms which regulate the amount of food consumed by genetically skinny babies are well regulated. 

This is why the baby will vomit and/or poop more if you push him to consume more than he cares for by feeding him through a tube or other means.

Consequently, his body size won’t significantly change. However, even if this slightly increased his body size, it wouldn’t improve his health.

A genetically skinny newborn could also be malnourished. But, again, this is because of a feeding aversion.

Genetically slim kids are more likely to experience repeated pressure to eat when they don’t want to than normal-weight or plump children, which diminishes their urge to eat even when they’re starving.

Low Birth Weight

Your child might remain small for the first few months of his life or even longer if he was born at a low birth weight due to premature or early delivery.

You should also remember that even though babies are born at low, average, or high weights, their development can also change. So a modest slip on the growth curve can be normal in the two-step-forward, one-step-back dance of a baby’s growth. 

However, consult your doctor whenever you notice this is happening. This is because sometimes a regression on the growth curve can be an indication that there is something wrong.

Bottled-Fed Versus Breastfed

Now let’s talk about the way you feed your baby. Although it may appear stereotypical, how a frequently bottle-fed and a breastfed baby gains weight differs during the 1st year of life. 

According to a study from 2012, infants gained less weight at three, five, seven, and twelve months the more breast milk they received. On the other hand, the weight of bottle-fed babies increased as the number of bottle-feedings they took increased. 

However, this doesn’t necessarily mean bottle-fed babies are overweight or that breastfed babies are underfed. This is why it is essential to have other ways and indicators to know when your baby is underfed or overweight.

It is essential that you know that your child can have trouble sleeping even when he is well-fed. This could be a sleep association issue, making the baby grumpy and irritable. 

Additionally, constipation can occur depending on the type of food or formula you feed him.

Here are some of the signs which can help you know when your child is developing typically and reaching their developmental milestones:

1. If There Are Baby Rolls On Your Child

One of the ways to tell when your baby has a good weight and is developing normally is the presence of baby rolls on their back which gives them the appearance of a fluffy little loaf of bread.

However, you shouldn’t be worried if your child doesn’t possess those baby rolls. Likewise, it is nothing to be worried about if your doctor reports that your child is developing well.

2. If His Poops Are Regularly Wet

Bowel movements are another indicator you should check because you can know your baby’s developmental progress.

For example, suppose your baby is developing normally if he has about six times daily (as long as it isn’t severe diarrhea). But if he has a number significantly below or over 6, it is a sign that something might be wrong.

You can also tell when something is wrong by inspecting the poop. If he passes a lot of dry poop or if the consistency, color, and mucous content change. Even though the presence of black specks in stools might not be harmful, it is still best to speak with a doctor if you find them. 

Some actions are needed if your child is experiencing any of these issues in order for them to continue being healthy.

How To Know When Your Baby Is Underweight

There are several methods to know when your baby is underweight. You can use any of these two methods.

1. Body mass index (BMI) for age.

2. Weight-to-length standard deviation scores (SDS)

If You have confirmed that your child is underfed, the next thing you can do is determine if he is a genetically skinny or underfed baby. You can also read more about other things you could do.

Making this distinction is crucial since it will determine whether you need to take any action and the kind of action you need to take. But don’t be alarmed because differentiating the two is relatively easy. 

How To Know When Your Baby Is Underfed

You will notice this in his behavior when a neurologically healthy child is underfed. His behavior will change depending on his level of malnutrition. Your baby could be severely malnourished or underfed.

Here are the signs that will help you to know when your baby is underfed: 

  • If he rarely smiles and is irritable 
  • If he is not active when he is awake, even though he is alert 
  • If he wants you to carry him all the time and cries whenever you try to put him down.
  • If he keeps demanding your full attention. You can be able to tell when he keeps crying.
  • If he has trouble sleeping and staying asleep.
  • If he doesn’t wet the diapers up to 4 times a day.
  • If he only has bowel movements once in three days or more. And if he does not have constipation when he is fed formula.

Malnourished babies are often weak. This is because their body is trying to conserve as much 

energy as possible.

Here are some signs or indications that your baby is severely malnourished: 

  • If he rarely asks for food and does not display any signs of hunger.
  • If he is always weak or tired.
  • If he usually nods off before finishing his meals.
  • If he sleeps for a long period.
  • If you need to wake him up to eat and if he isn’t alert.
  • If he doesn’t wet the diapers about four times a day.
  • If he rarely has bowel movements. He may have a tummy ache or be experiencing “starvation stools” or infrequent, green, slimy feces.

If you feel your baby is underfed or underweight and want to help him increase his size, it is best to check their BMI first.

If his BMI is within the normal range, there is no need to worry because he is perfectly healthy. However, if you are still concerned or need clarification on the result obtained from the BMI, feel free to consult your pediatrician.

Always remember that overfeeding could lead to obesity and health issues on the road, neither of which you want for your child. This is why we advise you to overlook his weight and focus on his progress alone. 

Other Issues that can lead to malnourishment & growth problem 

Several factors contribute to failure to thrive, and they may occasionally be treated with special diets or go away without you carrying out any action.

1. Gastroesophageal Reflux Disorder (GERD).

When your child has this disorder, his appetite & eating habits will be disrupted because this disorder(GERD) causes the stomach content to flow back up through the esophagus. This causes a burning sensation because of stomach acid in the esophagus. 

Consequently, your skinny baby may even refuse to eat.

This backup flow of stomach acid to the esophagus occurs when the lower esophageal sphincter becomes so weak that it cannot seal entirely because the digestive system is still developing.

It is usual for reflux to disappear when your child is about one year old.

A specific acid reflux formula is sometimes used by parents who are formula-feeding their children. But surgery may be required in extreme situations.

2. Celiac Disease

Celiac disease is another thing that can lead to the malnourishment of your baby. 

This disease is a more common problem that has recently received more attention in the medical community.

It makes your child intolerant to gluten. It is a problem because gluten is a prevalent ingredient in many foods available on the market, and everyone widely consumes it regularly. 

Nevertheless, it isn’t much of a big deal if your baby has a gluten intolerance. 

This is because the market has changed significantly recently, and a gluten-free option makes it possible for those with celiac disease to have a trouble-free, healthy life.

This was possible because the problem is recognized to contribute to a certain degree of malnutrition & failure to flourish, which was why the problem and solution were discovered.

3. Any Serious Illness Or Heart Condition

There is nothing worse for parents to think about than the idea of their child having a chronic medical condition that doesn’t have a recognized treatment.

Your baby’s diet and general appetite might be negatively impacted by conditions including cerebral palsy, cystic fibrosis, diabetes, several types of heart disease, and others, which can ultimately cause problems with food consumption.

All you can do as a parent is help him manage the symptoms to lead a better life because there are no recognized treatments for such diseases.

You must give him some diet or treatment to help him maintain a healthy weight.

Frequently Asked Questions

How Can I Make My Skinny Baby Fat?

You must avoid artificial sweeteners & sugars if your baby is over six months old and starting to wean. It will help if you choose healthy fats like nut butter, fish, and avocados instead.

Is It A Bad Thing For My Baby To Be Skinny?

Some infants are referred to as “genetically lean,” which means that their genetics will make them appear leaner; nevertheless, this does not imply that they are underweight or underfed.

However, in some cases, malnutrition may cause your baby’s skinny appearance. Still, you can solve this just by seeing your doctor or a medical dietician to help you deal with the situation.

What Happens When Your Baby Skips A Feed?

Parents tend to be concerned when their baby skips a meal because they are sleeping or occupied with something else; however, this is nothing to be worried about because your baby will always let you know the next he feels hungry.


Crying is the only way your child communicates a problem to you. If he can’t express his demands, he will feed less and be irritable (act dissatisfied) more frequently.

Remember that a baby’s appearance is primarily determined by genetics, so being underweight does not necessarily suggest they are malnourished.

The severity of the causes of a thin infant varies widely.

Most of the time, the causes are benign and are easily rectified with little instruction.

However, every child must be unique in their way. So, it would help if you didn’t compare your children to those of others because what works for one child may not work for another, and the reverse is also true.

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