How Genetics Influence Baby Weight More Than Most People Realise

BySamradni Pradhan
Updated: 26 Dec 2025, 8:00 pm

The role of genetics in influencing your baby’s birth weight is far bigger than you realise. You may hear advice from your elders to eat healthy or stay active, but your baby’s natural growth depends on the traits they inherit from their parents. You can manage your expectations and reduce unnecessary stress after understanding how genetics affects baby weight. You will also be better prepared for discussions with your doctor during pregnancy.

Pregatips
You may hear all kinds of predictions from your family about your baby when you are pregnant. They may comment on your baby being big or small, depending on what you eat. But that is not true. In fact, your baby’s weight is determined by something far beyond your control. The answer is genetics. Yes, the genes you and your partner carry affect your baby's weight. Understanding why it matters will help you set realistic expectations for your baby and avoid worrying about their weight.
In this article:


What Makes Genetics So Important for Baby Weight?


If you were pregnant before, you may notice that your baby's size differs in each case. Their birth weight may also vary. But why? The answer is your baby’s genetics, which they inherit from you and your partner.

Studies show that genetics make up to 40–70% of a baby’s birth weight. It shows that a baby’s natural size depends on family patterns, whether you or your partner were small, average, or heavier at birth.


Which Genetic Factors Influence Baby Weight?


Here are the inherited traits that may affect the growth of your baby:

  • Do parental birth weights matter?

Your baby will be born heavier if you or your partner also weighed more than 3.5 kg at birth. Similarly, the baby may be born small if both parents were also small at birth.

  • Does height influence growth?

Taller parents have babies with longer bones and greater growth in all aspects, as they follow family trends. It does not necessarily mean a big baby.

  • What about metabolism?

The baby may inherit metabolic traits from their parents, which can affect how they gain weight. Metabolic activity can be faster or slower in either the parent and, so, in the baby.

  • Can inherited medical conditions play a part?

The risk of gestational diabetes, thyroid tendencies, or insulin sensitivity can influence your baby's weight, as it gets passed down.


How Much Does Your Diet Actually Matter?


Although your health is an important factor, it does not decide your baby’s weight alone.

  • A healthy mother will have a baby whose size follows their genetic pattern.
  • Overeating does not necessarily mean a large baby.
  • Under-eating is more harmful because it can restrict growth and affect your health.


Why Do Some Babies Grow Faster or Slower in the Womb?


The major reason is that the babies grow at their own pace, which can make you wonder about the following points:

  • Is it normal if my baby measures ahead or behind?

Ultrasound measurements can vary by 10–15%, and genetics can explain the reason for your baby appearing larger or smaller at different stages.

  • Is it always a sign of a problem?

Doctors check growth over time, not just one scan. You can be reassured about things going well if your baby’s growth curve shows a steady rise.


Can the Environment Still Influence Genetic Growth?


Genetics are the base, but your environment affects how those genes express themselves.

  • How does your health impact growth?

High blood pressure or anaemia may reduce placental blood flow, leading to slower growth regardless of genetics.

  • What about lifestyle factors?

Smoking, poor nutrition, or severe stress can also interfere with normal development.

  • Does emotional well-being matter?

Constant stress may trigger hormonal changes that affect placental function. You can create a positive environment for your baby by taking time to rest and talking to your partner about your emotional needs.


When Should You Worry About Baby Weight?



What Can You Do to Support Healthy Baby Growth?


All you need to do is take care of your lifestyle habits.

  • Eat fruits, vegetables, and whole grains
  • Take prenatal supplements as prescribed
  • Stay active by walking or doing prenatal yoga
  • Attend all the checkups to track growth
  • Manage stress through deep breathing, light stretching, or talking to loved ones
  • Stay hydrated, especially in hot or humid climates


What Should You Do Next? A Quick Checklist


  • Remember that genetics guide your baby’s natural weight
  • Focus on healthy habits instead of weight
  • Keep a record of your scans and doctor discussions
  • Ask your doctor how your baby’s growth curve looks
  • Share concerns with your partner for emotional support


The Emotional Side: Why This Knowledge Helps You Relax


Pregnancy isn’t only about challenges; it can be enjoyable too when you have supportive family and friends. You can take a sigh of relief knowing that genetics plays a bigger role. The first thing you must do is stop comparing your bump size with others. Also, try not to stress about your baby’s weight after hearing comments about them.

Your baby’s weight is defined by genetics more than the choices you make every day. A healthy lifestyle supports growth, but it does not override your baby’s inherited traits. So, avoid unnecessary stress over small things and discuss your concerns with your doctor rather than your relatives. Remember that your baby is growing at their own pace, which is not based on societal expectations.

FAQs on How Genetics Influence Baby Weight More Than Most People Realise?

  1. Can I change my baby’s weight through diet?
    Your baby’s natural size depends on genetics, so there is not much you can do other than eat healthy for their proper growth.
  2. Why don’t all my babies have the same birth weight?
    Each pregnancy is different. Your babies may grow differently depending on factors like how well the placenta works that time, your health during that pregnancy, and how each baby’s growth pattern naturally varies. It’s completely normal for siblings to be born at different sizes.
  3. Does a bigger baby mean a more difficult delivery?
    The most important factors guiding the delivery are your pelvic structure, baby’s position, and medical support.
Medically Reviewed By:
Dr Swetha S. Reddy, Consultant Gynaecologist and Obstetrician, SPARSH Hospital, Yelahanka