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Why Underweight Babies Often Have Faster Heartbeats

BySamradni Pradhan
Updated: 14 Jan 2026, 6:00 pm
Medically Reviewed by Dr. Megha D. Kadam, Consultant – Neonatology & Paediatrics, SPARSH Hospital

When your baby arrives smaller than expected, your mind goes into overdrive. You notice things most people wouldn’t even think about, especially how fast their tiny heart seems to beat. It can make you anxious, but in many cases, this quicker rhythm is your baby’s way of coping in the early days. Their bodies are learning, warming up, adjusting, and catching up. Once you understand why it happens, it becomes much easier to breathe, care, and know when something actually needs a doctor’s opinion.

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Your baby’s first days are confusing. Everything is new, you’re tired, emotionally labile, and monitoring your baby for hours, half in awe and half in fear. And when your baby is underweight, you automatically pay extra attention. One of the first things that often stands out is how quickly their heart seems to beat when you place your hand on their chest. It can be startling. But babies don’t follow the same rules as adults. Their bodies work at a different speed altogether, and small babies work even harder. Knowing that makes those rapid little beats feel somewhat less frightening.
In this article:

What Does a “Fast Heartbeat” Mean in Babies?


A newborn’s heart is naturally busy.

  • Most babies beat between 120 and 160 bpm
  • Preterm and low birth weight babies can go up to 160–180 bpm, which is the normal rate for these babies.
So even if their heartbeat feels rapid, it doesn’t automatically mean something is wrong. Their body is just doing its best to keep pace with everything happening at once.

Why Do Underweight Babies Have Faster Heartbeats?

1. Their Body Compensates for Lower Fat and Stores Energy
A smaller baby usually has fewer energy reserves. Less fat, less stored fuel. So their heart takes on extra work, pumping faster to deliver oxygen and nutrition to the rest of the body. It’s almost like their body saying, “Alright, I’ll pick up the slack.”
If you ever notice sudden changes, such as a drop in feeding or your baby seeming unusually quiet, consult your physician.

2. Their Small Body Needs More Oxygen
Underweight babies have bodies that tire easily. Their lungs are small, their haemoglobin levels are low, and their breathing can be shallow or slightly irregular when their brain is still immature. To ensure oxygen delivery, the heart increases its rate.

3. Challenges with Temperature Regulation
This is something many parents don’t expect: small babies lose heat fast. Their bodies lack substantial subcutaneous fat to retain heat.
When they feel cold, their metabolism increases to generate heat, and their heart rate rises accordingly. You may notice their heartbeat increasing slightly when you unwrap them or when the room feels cold. Skin-to-skin contact or warm layers help substantially.

4. Immature Organ Development Affects Heart Rhythm
If your baby came early, their developing body is still synchronising. Their nervous system is figuring out signals, lungs are adjusting, and the heart is still learning how to maintain a steady rhythm. Because of this, the heart can beat faster than you expect.

5. Stress, Hunger, or Discomfort Raise Heart Rate Quickly
Small babies get hungry quickly. Their stomachs barely hold anything. They also tire more easily, so crying, hunger, or even a bit of frustration can raise their heart rate, a common physiological response. Fortunately, it settles once your baby is held, fed, or soothed. These jumps are often normal responses, not signs of illness.

Is a Fast Heartbeat Always a Sign of a Problem?

No, not always. Many underweight babies have quick heartbeats because they’re working harder to adjust to the world. It’s how their bodies cope.

If the heartbeat goes above 180 bpm and stays there while your baby is calm, or you see other worrying signs like not feeding well, lethargic, reduced urine output or forehead sweating while feeding or the baby struggling to feed continuously, it’s time to contact your doctor.

What are the Warning Signs to Watch For?

Call your doctor if a fast heartbeat comes along with:

  • Breathing trouble
  • Refusing feeds
  • Blue tint on lips
  • Too much sleepiness
  • Cold hands and feet
  • Low body temperature
  • Non-stop crying or irritability
  • Forehead sweating
  • ⁠Unable to feed continuously and taking frequent breaks (suck rest suck cycle)
  • Reduced urine frequency
These signs mean your baby needs support quickly.


How You Can Support an Underweight Baby at Home

1. Keep Your Baby Warm
Smaller babies lose heat fast. Keeping them warm is simple but powerful. It causes them to expend energy on growth rather than on maintaining temperature.
Try:
  • Skin-to-skin time
  • Keeping the room at 24–26°C
  • Soft layers

2. Feed Them Frequently
Their stomach capacity is smaller, hence they need to be fed more frequently, at least 2-3 hourly initially and gradually increase the gap as the baby grows.

You can:
  • Feed every 2–3 hours
  • Look for 6–8 wet nappies
  • Follow your doctor’s advice about milk fortifiers

3. Track Their Weight
Weight gain gives a clear picture of your baby’s progress. Even a small increase each day, 20–30 grams, shows things are moving in the right direction.

4. Provide Emotional Comfort
Never underestimate this. Babies feel safe when they feel you. Your voice, your warmth, and your hands all help your baby relax. Their heartbeat responds to your closeness more than you may realise.

5. Keep Up with Doctor Visits
Underweight babies often need more frequent check-ups. These visits help track:
  • Heart rate
  • Feeding patterns
  • Growth
  • Any signs of nutritional deficiencies
Regular monitoring makes a big difference.

What Should You Do Next? A Simple Checklist

  • Watch how your baby feeds
  • Keep them warm
  • Track nappies
  • Follow your doctor’s feeding plan
  • Notice breathing and colour changes
  • Include skin-to-skin time daily

How Your Family Can Help You Cope

Caring for a small baby is draining. You might feel guilty, worried, or constantly alert. That’s normal.

Your family can support you by:

  • Helping at night
  • Taking over chores
  • Going with you to appointments
  • Watching the baby when you rest

And most importantly, maintaining the mother’s nutrition and supplements in check, which will help in the faster recovery of the mother and also milk output. You don’t have to power through everything alone.

A rapid heartbeat in an underweight baby is often the body's response to a new and demanding environment. Once you understand the reasons behind it, caring for your baby feels a little easier. With warmth, good feeding, steady comfort, and regular medical guidance, your baby can grow stronger every day.

FAQs on Why Underweight Babies Often Have Faster Heartbeats

  1. Do underweight babies always have heart problems?
    No. Many small babies have fast heartbeats without any heart issues. As they grow, the rate usually steadies.
  2. Should you check the baby’s heart rate daily?
    Not unless your doctor tells you to. Watching feeding, behaviour, and breathing is usually more helpful.
  3. Can an underweight baby catch up later?
    Yes. Many do within 6-12 months, especially with proper feeding and regular check-ups.
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