Why Is My Baby Underweight?

Your baby is skinny. You have been told he’s underweight. Should you be worried? Maybe! Maybe not! It's important to know there’s a difference between an underfed baby, which is cause for concern, and a genetically lean baby, which is not a reason to worry. This article explains the difference.

Is baby underweight?

Is your baby underweight or is it that you think he is?

In general, people living in western societies (in particular those that speak English as a first language) have a distorted view on what a healthy baby looks like. Most people can’t recognize when a baby is overweight simply by looking at him. Chubby babies, even those who are overweight and obese, are considered to be healthy. Parents of normal weight babies are told their baby is thin. Lean babies are assumed to be underweight.

There are ways to determine if a baby is underweight. The two most common methods are:

Weight-to-length standard deviation scores (SDS)
Body mass index (BMI) for age.

If you’re not sure if your baby is underweight, follow one of these links and find out if this is the case or not.

If your baby is not underweight, and instead his weight is in a normal range, you might need to adjust your perception about what a healthy baby looks like.

If your baby is underweight, the next step is to identify if he’s an underfed baby or a genetically lean baby. This is a very important distinction to make because it will determine what course of action, if any, you need to take. It’s easier than you might think to tell the difference.

Genetically lean baby

A genetically lean baby inherited ‘skinny’ genes from one or both parents. A parent or close family member has a history of being underweight or diagnosed as ‘failure to thrive’ as a baby, and remained skinny as a child and lean as an adult.

Baby could be born skinny, perhaps diagnosed as IUGR (intrauterine growth restriction) at birth. Or he might have be born normal weight and become progressively leaner during the early months.

Genetically lean babies are underweight babies when using medical criteria. Baby’s length is as expected given the size of parents and other family members. His weight sits 2 or more percentile curves lower than his length on an infant growth chart.

A genetically lean baby will show signs of being well fed, such as...

He demands regular feeds. (Many well-fed babies are content to wait until they’re offered a feed.)
He appears satisfied at the end of the feed. (He’s likely to get upset if you try to make him eat more than he wants.)
He engages with others and it’s easy to get him to smile.
Between feeds he’s a happy baby except when tired or bored.
When awake, he’s alert, active and energetic.
He’s inquisitive and interested in his surroundings.
He sleeps well.
He has five or more wet diapers each day.
He has regular bowel motions. This can range from 3 times a day to once every 3 days if he’s formula-fed. Or more or less if he’s breast-fed.
He’s achieving all developmental milestones.

Basically, the only thing he’s missing is a thick layer of body fat!

You can’t fatten a genetically lean baby. He will willingly eat what his body needs and no more. Homeostatic mechanisms that protect babies from overfeeding are highly tuned in the case of genetically lean babies. Making baby eat more than he wants by force or tube feeding will cause him to throw up more and/or poop more. It won’t make a significant difference to his body shape. And even if this was to marginally increase the amount of fat on his body, it won’t make him healthier.

Note: Well-fed baby can experience sleeping problems due to other reasons, such as a sleep association problem, and become irritable as a result of sleep deprivation. Also a well-fed baby could become constipated due to the type of formula or solids given.

How to tell if your baby is underfed

If a neurologically healthy baby is not getting enough to eat, he’s going to let you know though his behavior. The type of behavior will differ depending on the degree to which he’s lacking nourishment. A baby could be underfed or he could be severely malnourished.

Signs that a baby is underfed or underfeeding include:

He’s often cranky and seldom smiles.
When awake, he’s alert but not active.
He wants to be held constantly. He fusses when put down.
He demands full attention.
He has trouble getting to sleep and staying asleep.
He has four or less wet diapers (based on milk feeds alone).
He has infrequent bowel motions, i.e. once every 3 days or longer. If formula-fed he could become constipated.
You can’t tell if he’s reaching his milestones or not because he seldom leaves your arms except when sleeping.

A malnourished baby is a weak baby. His body will try to conserve energy. Signs that a baby is severely malnourished include:

He doesn’t show signs of hunger and rarely demands feeds.
He sucks weakly.
He often falls asleep before completing feeds.
Between feeds, he’s listless and lethargic.
He sleeps for prolonged periods.
He has to be woken for feeds, and it’s difficult to awaken him.
He has four or less wet diapers per day (based on milk feeds alone).
He has infrequent bowel motions. He might be constipated or pass ‘starvations stools’ i.e. infrequent green slimy stools.
You can’t tell if he’s reaching his milestones or not because he’s so sleepy.

You can fatten an underfed or malnourished baby. However, to do so you may need to first identify the cause. Seldom is this because baby is not offered enough. The most common reason for babies to not willingly eat enough is because they have developed a feeding aversion. (See Underfeeding for other reasons.

It is possible for a genetically lean baby to also be underfed baby. Usually this is due to a feeding aversion. Genetically lean babies are at increased risk of developing a feeding aversion compared to normal weight and chubby babies, solely because they’re more likely to be repeatedly pressured to feed against their will, which then causes them to not want to eat even when hungry.

Misdiagnosis of genetically lean babies

Genetically lean babies’ growth is often mistaken as poor growth when compared against average weight gain figures for babies of the same age. Many genetically lean babies are misdiagnosed as ‘failure to thrive’.

While it is easy to tell the difference between a genetically lean baby and underfed babies, many health professionals don’t make the distinction (by asking parents relevant questions about baby’s behavior and wet and soiled diapers) because they assume an underweight baby = underfed baby.

Consequence of misdiagnosis for genetically lean babies

Failure to recognize a genetically lean baby means parents suffer needless anxiety. Poor feeding advice given as a result of the erroneous assumption that a genetically lean baby is underfed has the potential to cause baby to become averse to feeding. See consequence of baby growth mistakes for how this happens.

Some genetically lean babies end up being tube fed due to a series of events that first began due to a lack of understanding that genetically lean babies don’t gain average weight. And they don’t carry the amount of body fat that other babies do. Aiden’s case below is an example.

Case study

Aiden was born average weight and length. From the time of birth his weight gains appeared to be poor. As a result, his mother, Evelyn, lost confidence in her ability to breastfeed. When Aiden was 4 weeks, she decided to switch to bottle-feeding.

Up until Aiden was 8 weeks old Evelyn found that with a little perseverance she could make him drink the recommended amount of milk. But then he started to leave milk in the bottle and resist her attempts to make him drink more. His weight gains again dropped below expected for his age. Worried, Evelyn would spend most of her day and night trying to make him eat the recommended amount. Each feed would last over an hour. What he did not drink while awake, she would try to make him take while sleeping. Evelyn stated her entire day revolved around trying to make sure Aiden got enough to eat. But as much as she tried she could no longer get him to eat the recommended amount.

By 3 months of age Aiden was classified as ‘failure to thrive’. As the weeks passed a number of reflux medications and formula changes were tried without improvement in his feeding or growth, and the decision was made to tube-feed him. The plan was to let him eat what he wanted and then top him up to the recommended amount of formula through the tube. Within less than a week he stopped feeding orally and was totally tube fed.

He continued to be fully tube fed until he was 6 months old. During this time he vomited often. If he was not held in a parent’s arms he was swaddled, even while awake, to prevent him from pulling out his tube. He was fed why lying in baby rocker. Evelyn felt she could not move him or play with him after feeds for fear he would vomit. He was switched to high-energy formula to provide more calories in a lower volume and given Erythromycin to increase gastric emptying time (the time it took for milk to empty from his stomach), but still he vomited. In an attempt to reduce vomiting, his feedings were slowed down. He was attached to the pump for 16 hours a day. There was only 1 hour’s break between the end of one feed and the start of the next. This meant his family could not leave the house for longer than an hour at a time. He had frequent large, foul smelling bowel motions each day. He was often gassy and uncomfortable and slept poorly. But he did gain some weight. His weight slowly edged toward the lower end of a normal range in relation to his length. Aiden and his family paid a high emotional cost for these few ounces of extra body fat.

At 6 months of age Aiden’s parents consulted with me about the possibility of weaning him from tube feeds to bottle-feeding. At this time I suspected he was averse to bottle-feeding due to being forced-fed in the past and currently being ‘encouraged’ (unsuccessfully) to take a bottle using gentler forms of pressure. So this needed to be addressed. I recommended they switch him back to normal strength formula. He was successfully weaned over a period of two weeks. After tube weaning Aiden would get excited at the sight of the bottle when he was hungry, grab it, bring it to his own mouth, feed contently, and stop sucking in a relaxed state when he was done. But he only ate about three quarters of the calories his parents had been told he needed by his doctor. Worried, they were advised by a dietician to give him high-energy milk once again, but this resulted in a drop in the amount he would eat. He consumed roughly the same number of calories either way. Proving to be of no benefit, they returned him to regular strength formula so that he would receive more fluids.

Over the next month Aiden gained very little and his BMI dropped below the range considered as normal, placing him back into the underweight category. His parents were understandably worried. Yet despite not drinking or gaining as much as expected Aiden was far happier now that he was deciding how much he would drink. He seldom vomited. He was no longer gassy, and slept better. He was more energetic. He was laughing for the first time, and interacted with his parents and others more than he did while being tube fed. He also developed a few new skills, rolling and then sitting, in rapid succession as a result of no longer needing to be restrained to prevent him from pulling out the tube and from being placed on the floor to play without the fear of him vomiting up his feed.

Even though Aiden’s BMI meant he was in the underweight category, he had fat on his body, just not as much as he had previously when tube fed or as much as the ‘average’ baby has. Apart from being lean, Aiden displayed all the signs that he was well fed.

Both Aiden’s parents were lean; his dad, Marcus, in particular. I asked about other family members. His father’s parents and father’s siblings were all very lean. Interestingly, Marcus had been diagnosed as ‘failure to thrive’ as a baby, he remained lean as a child and was now lean as an adult. Apparently he has been considered to be underweight all of his life.

Aiden too was genetically inclined to be lean. Over the next few months his weight continued to follow a growth curve 3 percentiles below his length, medically classified as underweight, but he remained a happy energetic baby who was into everything, and it became clear that he is naturally inclined to be leaner than most other babies.

Written by Rowena Bennett.