Is it normal for one twin to be smaller?

Yes. Twins often differ in size at birth and through infancy, sometimes because of how they shared space and the placenta. A steady gap where both babies grow well is usually nothing to worry about.

Will the smaller twin catch up?

Many twins narrow the gap over the first year or two, but not all, and that can be fine. What counts is that each twin grows steadily along their own curve. Your health visitor can reassure you at reviews.

Should I compare them on the growth chart?

Plot each twin separately and follow each one's own line. Comparing the two babies directly is less useful than checking that both are tracking steadily for their own size.

When should I be concerned?

Ask a professional if either twin stops gaining, drops across centile lines, has fewer wet nappies than usual, or is feeding poorly or seems unwell. Trust your instinct and get them checked.

Could the smaller twin be missing feeds?

It happens, especially if one twin is a stronger or hungrier feeder. Tracking each baby's feeds helps you make sure the smaller one is getting full, regular feeds rather than being out-competed.

One twin smaller than the other

One twin is much smaller than the other

Quick answer

A size difference between twins is very common, especially at birth, and many twins even out over the first year. What matters is not that they match each other but that each twin follows their own growth curve steadily. Track each baby separately and raise it with your health visitor or doctor if either one flattens off, drops across centile lines, or is feeding poorly.

You lay them next to each other and the difference is obvious. One twin is noticeably bigger, one is noticeably smaller, and every weigh-in turns into a worry about the little one.

A size gap between twins is one of the most common things twin parents notice, and much of the time it is nothing to fix. Twins share space and, in some cases, a placenta, so starting at different sizes is normal.

Track each baby on their own line

The trap is comparing the twins to each other. On a growth chart, each baby has their own curve, and the reassuring picture is each twin gaining steadily along their own line over weeks. Two different lines, both climbing, is a healthy pattern.

Know when to ask

Get a twin checked by your health visitor or doctor if they stop gaining, drop down through the centile lines, have fewer wet nappies, or feed poorly. A steady smaller twin who feeds well and has normal nappies is usually thriving at their own size, but you should always follow professional advice for your babies.

Frequently asked questions

Is it normal for one twin to be smaller?: Yes. Twins often differ in size at birth and through infancy, sometimes because of how they shared space and the placenta. A steady gap where both babies grow well is usually nothing to worry about.
Will the smaller twin catch up?: Many twins narrow the gap over the first year or two, but not all, and that can be fine. What counts is that each twin grows steadily along their own curve. Your health visitor can reassure you at reviews.
Should I compare them on the growth chart?: Plot each twin separately and follow each one's own line. Comparing the two babies directly is less useful than checking that both are tracking steadily for their own size.
When should I be concerned?: Ask a professional if either twin stops gaining, drops across centile lines, has fewer wet nappies than usual, or is feeding poorly or seems unwell. Trust your instinct and get them checked.
Could the smaller twin be missing feeds?: It happens, especially if one twin is a stronger or hungrier feeder. Tracking each baby's feeds helps you make sure the smaller one is getting full, regular feeds rather than being out-competed.

One twin is much smaller than the other

Track each baby on their own line

Know when to ask

Frequently asked questions

Sources

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