Babies Lose Weight at Birth: How Much and Why?

There are many reasons for this weight loss and in most cases it’s normal. Find out here why this happens!
Babies Lose Weight at Birth: How Much and Why?

Last update: 07 September, 2021

It’s a well-known fact that babies lose weight at birth. Specifically, a baby’s weight can drop considerably in the first few days of life. In fact, it can take about two weeks for this value to recover in its entirety and for the real growth process to begin.

It may sound somewhat contradictory and even if there are some diseases that can delay growth, in the vast majority of cases this is a natural phenomenon. If you want to know the reasons why this happens and to what degree this can affect your child, we invite you to read this article!

Do all babies lose weight after birth?

In general, weight loss in the first days of life is almost a constant in evaluated babies. This may vary depending on various personal and environmental factors. For example, a study published in the Revista Chilena de Pediatría (Chilean Journal of Pediatrics) in 2018 revealed that at 48 hours after evaluation there was an average loss of 6.85 %.

Newborn baby on a scale.

These values can change. However, most commonly such a situation doesn’t last more than a few days or, at most, two weeks. Even if there’s no well-established range for considering an exact value of weight loss as dangerous, it should be no more than 10% of the birth weight.

Find out more: Babies born overweight

Reasons why babies lose weight at birth

The transition from being in the womb to the outside environment may seem easy, but it’s not. The baby has to adapt to sudden changes in temperature, its cardio-respiratory system changes almost completely (including some small anatomical changes) and contact with other living beings and food sources has its consequences.

As for today’s topic, the fact that babies lose weight at birth is natural and can be explained by many of the factors mentioned above. The redistribution of fluids in the body plays a fundamental part, and this is what we’ll talk about next.

Urinary system

When the kidneys and the rest of the urinary tract begin to function outside the uterus, babies inevitably lose weight due to changes in the distribution of fluids in the body. The baby was accustomed to ingesting amniotic fluid during development. Therefore, this change also affects the pattern in which spontaneous urination occurs.

In fact, the adaptation that occurs after birth involves a major change in body water distribution that can affect weight. The youngest infants have a higher ratio of water to body mass, while adults gradually lose this ratio.

Gastrointestinal system

This goes hand in hand with feeding, and the exposure of the digestive tract to different substances —which in the first days of life should only come from breast milk—.  The subsequent defecation can also promote weight loss.

Meconium is the first stool of a baby. It has a very characteristic dark color, as well as a rather unpleasant odor. Did you know that the difficulty in expelling meconium is related to diseases such as cystic fibrosis? Fascinating, isn’t it?

Feeding, one of the reasons why babies lose weight

Breast milk provides all the nutrients necessary for newborns to grow properly, including the development of their immune system.

However, in the first few days of life, the amount of milk produced by the mother may be somewhat less than what she’ll produce later on. Despite this, there are some ways to increase breast milk production.

Colostrum is the first type of breast milk of a mother and has some differences in composition from well-developed milk. Over time, both the quantity and quality of this substance will promote adequate weight gain in the infant.

Feeding is one of the reasons why babies lose weight at birth.

What to do about it?

As you’ve seen, this weight loss is a natural phenomenon that shouldn’t alarm you too much. It’s important to remain vigilant during the first few days of life, especially paying attention to the appearance of other symptoms. For example, if your baby has difficulty sucking milk or having a bowel movement, there’s probably another associated health problem.

It’s not advisable to weigh your baby at every moment, as this could add to your distress. Just take a good look at your baby’s physical appearance, as well as his or her behavior. In case you have the possibility to weigh your baby, doing it every few days could be useful.

If you notice a large and prolonged loss (close to the 10% that we mentioned at the beginning), it’s advisable to see your pediatrician or neonatologist as soon as possible.

Be careful, but calm!

Yes, weight loss can be very distressing, especially for newborns. The important thing is to stay calm, as in the vast majority of cases it’s normal. If you detect any abnormality and you think it’s better to ask for an opinion, see your doctor.

All cited sources were thoroughly reviewed by our team to ensure their quality, reliability, currency, and validity. The bibliography of this article was considered reliable and of academic or scientific accuracy.

  • Gallardo M, et al. Descenso de peso en recién nacidos a término en las primeras 48 horas post natales. Rev Chil Pediatr 2018;89(3).

This text is provided for informational purposes only and does not replace consultation with a professional. If in doubt, consult your specialist.