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10 Factors That Affect A Baby's Birth Weight

10 Factors That Affect A Baby's Birth Weight

Your child's birth weight is one of the first things you'll be told when they are born - but not all babies weigh the same amount.

Knowing your baby's weight is not just useful to tell your friends and family though - it is also a clear indication of the baby's health.

But what factors affect birth weight? And what can be the problems of a low birth weight? Marsden Weighing are here to help explain.

Why is weighing babies important?

Tracking baby weight - right from birth - is a fundamental part of ensuring they are healthy and are growing as they should. Babies who are smaller at birth have a higher risk of birth complications, as well as an increased risk of developing diseases and other medical issues in later life. Newborns with a low birth weight also have an increased risk of dying within the first 28 days of their life.

That's why you'll find there's always an abundance of baby scales on maternity wards (and why health visitors always carry them around!) - as soon as a baby is born he or she will be weighed almost instantly. Baby scales have to be highly accurate and Class III Approved to make sure they provide a reliable weight reading with pinpoint accuracy.

We recently explained why weighing babies is important - including what can be spotted from weighing a baby at birth.

What is the average birth weight for a baby?

According to the World Health Organisation, a baby weighing less than 2500 g (5.5 lbs) is considered to have a low birth weight, regardless of how far along the pregnancy was. On the other hand, they state that healthy birth weight is between 3000 g and 3500 g.

Factors affecting birth weight

There are a number of factors that affect a baby's birth weight including:


A baby generally inherits half its genes from its mother and half from its father, so it makes sense that both have an impact on the genetics of the final baby.

The mother's height and weight impact the weight of the baby at birth - and the father's height and weight have an impact too. Some babies are small because it runs in the family.

However, some babies may take more after the mother or the father, rather than being an average of both.

Age of the parent

Evidence shows that teenage mothers are more likely to have a baby with a low birth weight (36%). Additionally, the risk of delivering LBW babies is almost twice as much for mothers who are younger than 20 years old.

Similarly women that are aged 35+ are linked to having babies of lower birth weight.

Number of babies

Another study indicated that a mother's first baby is more likely to have a low birth weight than their second child. This could be linked to the fact that the mother knows more about pregnancy than the first time around, but this is unconfirmed.

It is also worth noting that babies who are twins are born relatively smaller than singular children because the twins share a uterus. Additionally, the baby who comes out first is likely to be bigger than the baby who comes out second.

Length of Pregnancy

The gestational age at which a baby is born can also have a large impact. After all, a baby that is born at 32 weeks is going to weigh less than a baby that is born at 40 weeks

Babies put on their weight during the latter stages of pregnancy. Therefore, if the baby is born prematurely, then it will not have developed fully in the womb and is likely to be smaller than a baby born on term or post-term.

Mother's birth weight

While the father's height and weight are important, it appears that the mother's weight at birth plays a key role in the final weight of the baby. Women who are overweight are more likely to give birth to a larger than the average baby.

Similarly, women who have a lower weight during pregnancy are more likely to have a baby with a low birth weight (LBW).

Diet during pregnancy

Here are some factors that can be edited, such as the mother's diet during her pregnancy. If the mother under-eats, the required nutrients won't be passed to the child and they are more likely to be born underweight.

Mother's habits

Similarly, if the mother has smoking and drinking habits during pregnancy, the baby's birth weight can be reduced. Similarly, various types of drugs can also have a negative effect on a baby's birth weight


Slight differences between boys and girls can be observed. Generally, boys are slightly heavier.

Parent's medical conditions

Any medical issue during pregnancy can affect the weight of the infant. Conditions like anaemia, and high blood pressure also puts the baby at risk of being born underweight. Type 1 diabetes, type 2 diabetes, or gestational diabetes, on the other hand, can cause babies to be born overweight. If a mother's blood sugars aren't controlled during her pregnancy then that's the result.

A baby can also inherit hereditary diseases within the womb which can cause a low birth weight.


One final factor is the ethnic background of the parents. In the UK the birth weight of babies varies based on ethnicity, despite a baby's parents living in a similar socioeconomic environment.

This study found that the average birth weight for a white baby in the UK was 3416g, while an Indian baby, for example, had an average birth weight of 3072g. However, they believed that only 10% of these ethnic differences in birth weight were explained by socioeconomic factors.

For more information on the factors affecting a baby's birth weight, click here, or watch the video below.

What happens if a baby is born underweight?

The general rule of a low baby's birth weight is: The lower the birth weight, the greater the risk they face.

Problems of low birth weight include low oxygen levels, breathing problems, issues with maintaining body temperature, feeding problems and a greater risk of infection.

How to accurately check a baby's weight

If you’d like to keep track of your baby's weight at home, then Marsden Weighing offer a range of baby scales for home use that you can use in your own home.

For checking the baby's birth weight, the Marsden M-310 handheld scale is the perfect portable solution. The sling scale is ultra-lightweight and battery-powered meaning it can be taken anywhere - ideal for midwives.

The Marsden BAS-100HM Baby Scale is our best value baby scale for use at home. It comes with a built-in height measure so it can be used to measure a baby's height and weight, and can switch between kgs and lbs.

The Marsden M-300 is a Class III Approved Baby Scale. It has fine accuracy to 2g and battery power supplies more than 600 hours of operation. More information can be found in this case study.

Alternatively, the Marsden M-400 and Marsden M-410 have a baby tray that is removable to reveal a floor scale, suitable for toddlers, beneath. The M-400-80D is also available, which is fitted with a baby height rod.

To browse our full range of baby scales for home click here. For more information on any of our scales, call 01709 364296 or contact us here.

Further Reading

It is perfectly normal for babies to lose 10% of their birth weight, yet for new mothers weight loss can still be a concern. Find out how to measure how much breastmilk a baby is getting in this blog post.

After having your first baby, you’re likely to have a lot of questions, especially regarding breastfeeding. That's why we've compiled this blog post to answer all your questions.

In one of our previous blog posts, we discussed how weighing your baby is essential for a number of reasons. That's why it could be a good idea to purchase a baby scale for home use.