What is the normal weight difference between twins?

A difference of up to 15–20% is usually considered normal. Any difference greater than 25% may require additional follow-up with your doctor.

Can the smaller twin catch up in growth after birth?

Yes, many small twins grow rapidly after birth when provided with proper nutrition and care. Many catch up to the other twin within a few years.

Should you be concerned if one twin is significantly larger than the other?

Not necessarily. Some difference in size is normal, but if the difference is significant, your doctor will recommend the appropriate future steps (increasing ultrasounds or even early delivery if needed).

Why Twins May Have Different Birth Weights

Birth weight refers to how heavy your baby is at birth. It is an indicator used in assessing the growth and general health of your baby. The average birth weight for twins is less than that for single babies. Whereas a single baby usually weighs around 2.7–3.6 kg, twins normally come with weights between 2 and 2.5 kg each. However, sometimes there can be a significant weight difference between twins, which can leave you wondering.

Why Do Twins Often Have Different Birth Weights?

Unequal sharing of the placenta: If twins share the same placenta, i.e., in the case of monochorionic twins, one baby tends to get more nutrients and oxygen. This unequal supply affects growth, leading to varying birth weights. The medical term for this condition is selective intrauterine growth restriction (sIUGR).
Separate placentas with different efficiency: In the case of twins with separate placentas or dichorionic twins, one placenta may work better than the other. Accordingly, one baby may grow faster and healthier, while the other may develop more slowly.
Position of babies in the womb: The relative position of each baby in your womb also matters. For example, a baby with a bit more space to grow might actually grow more freely than one slightly restricted.
Differences in blood flow: Sometimes, the blood vessels connecting each baby to the placenta may not function equally well. In this case, one baby may receive less blood flow, which directly affects their weight.
Genetics and natural growth patterns: Just as two individuals born at different times may vary in their body size, twins, too, may naturally grow at different speeds. Genes do come into play, so one baby may be genetically bigger or smaller.
Twin-to-Twin Transfusion Syndrome (TTTS): If twins are in a shared placenta, they can have an imbalance in blood sharing. One twin (the donor) passes more blood to the other (the recipient). Hence, the smaller twin and the bigger twin undergo a distinct difference in weight.

Is It Normal for Twins to Have a Weight Difference?

Yes, some weight difference is quite common and usually not harmful. In fact, research indicates that 15–20% of twin pregnancies involve a noticeable weight difference between the babies. However, if the difference is too large (more than 20–25%), doctors will closely monitor the situation.

What Health Concerns Can Arise with Different Birth Weights?

In cases of twins with a small weight difference, most twins are born healthy, but in cases of a larger weight difference, closer monitoring may be needed:

The smaller twin baby may be at risk for low blood sugar, breathing issues, or feeding problems.
The larger baby may be affected by TTTS. This condition affects heart function.
Unequal growth may increase the risk of preterm delivery (before 37 weeks of gestation).

How Will Doctors Monitor Growth Differences?

If you are carrying twins, expect to have a few more ultrasounds. Some of these follow-ups may include:

Ultrasounds are performed every 2–4 weeks to monitor growth.
Doppler studies to measure blood flow.
Monitoring amniotic fluid levels around each baby.

The goal of this regular monitoring is to ensure that both babies grow as safely as possible.

What Can You Do to Support Healthy Growth?

1. Take Care of Your Nutrition

When pregnant with twins, your body is working twice as hard! You may need 300–600 more calories than you would with one. Focus on:

Protein-rich foods such as beans, eggs, chicken or fish.
Iron-rich foods to avoid anaemia, like lentils and spinach.
Drink lots of water.

2. Attend All Prenatal Appointments

Always attend your scans or check-ups, regardless of how well you feel. Your physician will have the best chance to direct you safely if they can detect a difference in growth early on.
Go to the doctor right away if you notice:

A sudden decrease in baby movements.
Severe abdominal pain.
Uncommon headaches or puffiness.

3. Emotional & Social Support

The experience of expecting twins is both amazing and daunting. You might want to:

Speak to a loved one or partner about your worries.
Discuss your feelings with other parents of twins or a support group.
Talk with your doctor about your mental health if you are feeling anxious.

What to Expect After Birth

The smaller twin may require additional assistance in the neonatal intensive care unit (NICU) for feeding or breathing.
The larger twin may settle more quickly, but both babies will be closely monitored.
Most twins with weight differences do fine and thrive with proper care.

Checklist: What Should You Do Next?

Consume a balanced diet with a moderate caloric intake and adequate protein.
Attend all prenatal appointments and follow your provider's advice.
Track baby's movements each day.
Mentally and emotionally prepare for possible NICU care.
Establish a support system after the baby is born.

Seeing differences in your twins' birth weights doesn't necessarily mean something is wrong. Small variations are expected, perhaps even acceptable. The focus should be on progressive monitoring, appropriate nutrition, and medical intervention when necessary. As long as one is supported adequately, both infants will thrive, regardless of their birth weight.
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FAQs on Why Twins May Have Different Birth Weights

What is the normal weight difference between twins?
A difference of up to 15–20% is usually considered normal. Any difference greater than 25% may require additional follow-up with your doctor.
Can the smaller twin catch up in growth after birth?
Yes, many small twins grow rapidly after birth when provided with proper nutrition and care. Many catch up to the other twin within a few years.
Should you be concerned if one twin is significantly larger than the other?
Not necessarily. Some difference in size is normal, but if the difference is significant, your doctor will recommend the appropriate future steps (increasing ultrasounds or even early delivery if needed).