What Determines Birth Weight

What Determines Birth Weight

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It’s funny how, as adults, we find being asked about our weight too personal and invasive. But with babies, this is one of the first questions people want to know. What’s up with that?

Here’s the short answer: it’s a way to know how healthy your baby is and how the mum’s doing without getting too personal.

On average, a newborn should weigh between 6 to 9 pounds. Of course, there’s a common belief that the heavier your baby is, the healthier they probably are and how they’re likely to develop, but that’s not always the case. Many factors can actually affect your baby’s birth weight.

What Determines Birth Weight?

From the baby’s conception to birth, factors have accumulated to determine how light or heavy they’d be.

Would they be in the normal range? Or would they fall below or above the limit? Let’s find out!


A baby takes 50% of their genes from their mum and another 50% from their father. Because of this, a baby’s birth weight can easily be influenced by the mum’s height and weight as well as the dad’s height.

Some babies are born big and large, or petite, depending on the genes they’ve inherited. This is more applicable for babies who are carried to full term.

For example, if the baby’s parents are both tall, chances are the baby would be born larger than an average-sized baby.

If the parents are more prone to obesity, the baby also has a chance of being born on the heavier side. In general, a huge chunk of the mum’s weight can reflect the baby’s average birth weight.

Baby’s Environment During Pregnancy

The prenatal development of the baby can also affect their birth weight.

What has the mum been eating? Was there a lot of stress or medical issues? Were there unideal pregnancy behaviours such as smoking or drinking? These are only a few questions that can answer what kind of environment the baby endured during pregnancy.

Obviously, the mum should have a healthy diet for her baby to grow well. When the mum under-eats, there’s a high chance that the baby won’t get the necessary nutrients to develop a healthy body. As a result, the baby could turn on the lighter side or even be underweight.

Another factor in prenatal development is the presence of stress or medical issues. Studies have shown that mums who are under a lot of stress don’t effectively transfer nutrients to their babies. As a result, babies are born lighter.

Also, mums who have existing medical conditions like anaemia or high blood pressure have babies who are born underweight. But if they have diabetes, babies can be born overweight.  

Gestation Length

pregnant woman

The time a foetus spends in the womb significantly affects the size and weight when born. A full-term baby and a premature baby would drastically differ in their birth weights.

On average, a preemie would only weigh about 5 pounds, sometimes even less. Babies who fall below the 10th percentile are considered “small for gestational age” or SGA. Don’t worry, though. This doesn’t mean that they’re automatically unhealthy. It simply means they’re small.

But when the baby is born late, you can expect them to be heavier. They may even be considered “large for gestational age” or LGA if they’re heavier than 90% of the babies at their gestational age.

Mother’s Age

Experts say that the most ideal time to give birth is between a woman’s late 20s to early 30s. Choosing to have babies outside of this age range, although possible, comes with greater risks.

Younger women, especially those who are below 20, have higher chances of having babies with low birth weight. On the other hand, women older than 35 years old had increased chances of birthing a big baby or foetal macrosomia

Pregnancy Type

Did you know that the type of the mum’s pregnancy can also affect the baby’s weight? 

If natural conception doesn’t work, some women seek in vitro fertilisation (IVF) to get pregnant. Because of the hormone treatment, babies conceived through IVF may be lighter than average.

However, by 17 years old, children born through IVF or natural conception have no remarkable differences in their average height and weight.

Number of Babies in the Womb


Another thing to consider when determining a baby’s birth weight is the number of babies in the womb. Generally speaking, there’d be less space for babies to grow if the mum is carrying twins or maybe even more babies. So, low birth weights are common for multiple childbirths.

For twins, it’s common to be born prematurely at 36 weeks, which we already know affects the baby’s birth weight.

For triplets, each baby can weigh less than half the size of a normal baby. So, this would mean their weights only range from 3 to 4.5 pounds. Birth weights for quadruplets and higher are even less than 3.3 pounds on average.

Ethnicity of the Baby

Lastly, the ethnicity of the baby can have a considerable effect on birth weight, too. It may actually play a larger role in determining the baby’s birth weight than their parents’ socioeconomic status. 

Recent research found that babies born to South Asian parents in the UK are generally lighter and have higher chances of being born with low birth weight. Also, Black Africans and Black Caribbeans have a 60% more chance of having babies with low birth weight. 

What Role Does the Birth Weight Play in Life?

Your baby’s birth weight can be a clear indicator of their health and development, especially during their first year. It’s also a great reflection of how much they’re nourished in the womb, but that doesn’t mean you should be wearing out your baby scales to check up on it.

Knowing whether or not your baby’s born within the healthy range can even tell you about possible developmental problems they may get during their childhood.

Sometimes, determining their birth weight can also help in figuring out their chances to develop different illnesses in their adulthood. 


Birth weight can definitely indicate some things, but it can’t indicate everything. It should only serve as a general guideline on how to manage your baby’s health.

Whether your baby falls below, above, or in between the average birth weight, it’s important to continue to nourish them as they go through a critical developmental stage.

Now that you know what determines birth weight, can you tell which factors influenced your baby’s birth weight the most?

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