Twin Development Milestones: Same Rate or Not?

Q: 2.2 Does the Smaller Twin Stay Smaller?

Not necessarily. Most smaller twins show catch-up growth in the first 6-12 months, especially if they were growth-restricted in utero but healthy at birth. However: – Some twins maintain a size difference throughout childhood — one is simply the larger child – Catch-up growth is most rapid in the first 6 months and typically plateaus by 12-18 months – The smaller twin should be tracked on their OWN growth curve, not compared to the co-twin

Written by the ChildBloom Pediatric Panel, FAAP | Medically reviewed July 14, 2026 | Last updated July 14, 2026

One of the first things people ask when they see twins is: “Are they at the same stage?” Are they rolling at the same time? Walking at the same time? Talking at the same time?

The short answer: probably not. And that is completely normal.

Twins are two separate individuals who happen to share a birthday. They have different genetic expressions (even identical twins), different birth experiences, different temperaments, and different strengths. Expecting them to reach every milestone at the same time is setting both the babies and yourself up for unnecessary anxiety and unhelpful comparison.

This article will walk you through what “normal” looks like for twin development, how to use adjusted age if your twins were born early, and when a developmental difference between the twins warrants evaluation.

PART 1: THE ADJUSTED AGE RULE —

1.1 What Is Adjusted Age?

If your twins were born before 37 weeks, their developmental milestones should be tracked using their adjusted age — their age calculated from their original due date, not their actual birth date.

Example: Twins born at 34 weeks (6 weeks early) who are now 6 months old chronologically are 4.5 months adjusted. You would expect them to perform like a 4.5-month-old singleton, not a 6-month-old.

1.2 How Long to Use Adjusted Age

For gross motor skills (rolling, sitting, crawling, walking): Use adjusted age until 24 months (2 years)
For fine motor skills (grasping, reaching, feeding): Use adjusted age until 24 months
For language (babbling, first words, sentences): Use adjusted age until 24 months
For social/emotional development (smiling, stranger anxiety, play): Use adjusted age until 24 months

After 2 years, most children born moderately preterm (32-36 weeks) have caught up developmentally, and adjusted age is no longer needed. Children born very preterm (<32 weeks) may need adjusted age tracking for longer — follow your pediatrician’s guidance.

1.3 Why This Matters for Twins

Most twins are born at least slightly early. If you are comparing Twin A (born at 35 weeks) to singleton developmental charts, you may think they are “behind” when they are actually right on track for their adjusted age. Using adjusted age prevents unnecessary worry and unnecessary referrals.

PART 2: BIRTH WEIGHT DISCORDANCE AND DEVELOPMENT —

2.1 The Size Difference

It is common for twins to have different birth weights — sometimes by a significant margin. A 25% or more difference in birth weight is called birth weight discordance and occurs in approximately 15-25% of twin pregnancies.

2.2 Does the Smaller Twin Stay Smaller?

Not necessarily. Most smaller twins show catch-up growth in the first 6-12 months, especially if they were growth-restricted in utero but healthy at birth. However: – Some twins maintain a size difference throughout childhood — one is simply the larger child – Catch-up growth is most rapid in the first 6 months and typically plateaus by 12-18 months – The smaller twin should be tracked on their OWN growth curve, not compared to the co-twin

2.3 Developmental Implications

A larger birth weight difference can be associated with: – The smaller twin reaching physical milestones (rolling, crawling, walking) slightly later due to less muscle mass – No significant difference in cognitive or language development in most cases – If the smaller twin was also more premature or had NICU complications, developmental delays are possible and should be monitored

The key point: track each twin individually. Do not compare Twin A to Twin B. Compare each one to the standard developmental charts (using adjusted age if appropriate).

PART 3: WHAT “NORMAL” VARIATION LOOKS IN TWINS —

Even full-term, same-weight twins will not reach every milestone simultaneously. Here is what normal variation looks like:

These variations do NOT indicate that one twin is “advanced” or “delayed.” They reflect individual differences in temperament, physical build, motivation, and opportunity.

Twins often develop in a complementary pattern: one twin may be the earlier talker while the other is the earlier walker. This is normal human variation, amplified by the fact that twins are constantly in each other’s company and may develop different niches.

PART 4: RED FLAGS — WHEN A DIFFERENCE IS NOT NORMAL —

While variation is expected, certain signs in either twin warrant prompt evaluation:

If EITHER twin shows any of these signs, request a developmental evaluation — even if the co-twin is developing normally. Each twin should be assessed independently.

PART 5: SUPPORTING BOTH TWINS’ DEVELOPMENT —

5.1 Individual Attention

Each twin benefits from one-on-one time with a caregiver. This does not require elaborate activities — it means holding, talking to, and playing with one twin while the other is napping or occupied. Even 15-20 minutes of individual attention per baby per day makes a meaningful difference.

5.2 Avoid Comparison

This is harder than it sounds. Family members, friends, and even pediatricians may compare the twins (“Oh, Twin A is so much bigger than Twin B!”). Redirect these conversations. Your response: “They are two different people with their own timelines.”

5.3 Tummy Time for Both

Tummy time is critical for both twins — it builds the neck, shoulder, and core muscles needed for rolling, sitting, and crawling. Do tummy time separately if one twin is more advanced or more frustrated. Use a mirror or colorful toys to engage each baby.

5.4 Read to Each Twin

Language development is driven by exposure to spoken language. Reading to each baby — even individually — builds vocabulary and comprehension. Narrate your day: “Now I am changing your diaper. Now we are going to the kitchen.” This “sportscasting” is one of the most effective language-building strategies.

PEDIATRICIAN’S TAKE —

The most important thing I tell twin parents about milestones is: track each baby individually, use adjusted age if they were born early, and stop comparing them to each other. I have seen parents become anxious because one twin rolls at 4 months and the other at 6 months. Both are normal. I have seen parents become proud of the ‘advanced’ twin and disappointed in the ‘slower’ twin. Both reactions are unhelpful. Your job is to provide each baby with the stimulation and support they individually need, and trust that they will reach their milestones in their own time.

WHEN TO CALL THE DOCTOR —

Either twin shows any of the red flags listed in Part 4
Either twin is consistently below the 3rd percentile on growth charts
One twin is significantly behind the co-twin in multiple developmental domains (motor AND language AND social)
You have concerns about either twin’s hearing or vision (these can affect development and are treatable)
Either twin was born very preterm (<32 weeks) — request a developmental follow-up program

Twin Development: What the Research Shows

Understanding the research on twin development helps parents know what to expect and when to seek support. Studies consistently show that twins tend to reach developmental milestones slightly later than singletons, particularly in language and motor skills. This delay is typically 2 to 4 months and is most pronounced in the first 2 years of life. Prematurity, which is common in twins, is a significant factor in developmental delays. Even twins born at 36 or 37 weeks may need adjusted age-based milestone tracking for the first 2 years. The language delay in twins is well-documented and is thought to be related to the fact that twins receive less one-on-one language input from caregivers because the caregiver’s attention is divided. Twins also develop a unique communication system with each other called twin language, which can delay the acquisition of standard language. The good news is that most twins catch up to their singleton peers by 3 to 4 years of age. The key is not to compare your twins to each other or to other children. Monitor each twin’s individual developmental trajectory and discuss any concerns with your pediatrician. Early intervention services are available for twins who need them, and the earlier the intervention, the better the outcome. The fact that one twin is ahead of the other in a particular area does not necessarily mean the other twin is behind — it may just mean that they have different strengths and developmental timelines.

Prematurity and Twin Development: Corrected Age Matters

If your twins were born prematurely, as most twins are, it is essential to use corrected age when evaluating developmental milestones. Corrected age is calculated by subtracting the number of weeks your babies were born early from their chronological age. For example, if your twins were born at 34 weeks, they were 6 weeks early. At a chronological age of 6 months, their corrected age would be 4 and a half months. Developmental milestones should be assessed using corrected age for at least the first 2 years. A premature twin who is not walking at 12 months chronological age but is walking at 12 months corrected age is developing normally. Using chronological age to assess milestones in premature babies leads to unnecessary worry and potentially inappropriate referrals. The corrected age approach gives premature babies time to catch up developmentally without being labeled as delayed. Even full-term twins should be assessed individually. Each twin has their own developmental timeline. It is common for one twin to talk earlier while the other walks earlier. Comparing twins to each other creates unnecessary stress and does not reflect their individual developmental trajectories. Focus on each twin progress toward their own milestones, use corrected age for premature twins, and celebrate each achievement when it happens, regardless of when the other twin reached that same milestone.

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Twin development milestones: Twin Milestones: Should You Expect Them to Develop at the Same Rate?

1.1 What Is Adjusted Age?

1.2 How Long to Use Adjusted Age

1.3 Why This Matters for Twins

2.1 The Size Difference

2.2 Does the Smaller Twin Stay Smaller?

2.3 Developmental Implications

5.1 Individual Attention

5.2 Avoid Comparison

5.3 Tummy Time for Both

5.4 Read to Each Twin

Related Articles

Twin Development: What the Research Shows

Prematurity and Twin Development: Corrected Age Matters

1 thought on “Twin development milestones: Twin Milestones: Should You Expect Them to Develop at the Same Rate?”

1.1 What Is Adjusted Age?

1.2 How Long to Use Adjusted Age

1.3 Why This Matters for Twins

2.1 The Size Difference

2.2 Does the Smaller Twin Stay Smaller?

2.3 Developmental Implications

5.1 Individual Attention

5.2 Avoid Comparison

5.3 Tummy Time for Both

5.4 Read to Each Twin

Related Articles

Twin Development: What the Research Shows

Prematurity and Twin Development: Corrected Age Matters

Related Posts

1 thought on “Twin development milestones: Twin Milestones: Should You Expect Them to Develop at the Same Rate?”