One twin crawls at 7 months. The other waits. By 18 months one strings two-word sentences and the other has six words total. Internet says fine. Grandparents say not fine. You're stuck in between. What research actually says:
- 30-40% of twin pairs show a 2-4 month gap on a major motor milestone (AAP)
- The lighter twin at birth usually catches up by 18-24 months
- Language gaps of 6-8 weeks are common; usually close by 30 months
- Identical twins share DNA, not timeline
- A 4-6 month gap on one domain by age 2 earns a pediatric conversation
Quick Reference: Twin Development Gaps
| Question | Short answer |
|---|---|
| Is a 2-month gap on milestones normal? | Yes, very common, no action needed |
| Is a 6-month gap normal? | Possibly, but worth raising at the next well-visit |
| Do identical twins develop the same? | No - same DNA, different timeline |
| Does one twin "hold back" the other? | Rarely; usually it's the slower twin developing at their own pace |
| When does the gap usually close? | Most motor gaps by age 2; language gaps by 30-36 months |
| Should I compare to singleton charts? | Yes for absolute milestones, no for twin-to-twin |
Why twins develop at different rates
Twins start unequal. Even identical twins, who share DNA, don't share the womb evenly. One twin usually has a slightly larger placental share, more amniotic space, or better cord position. That asymmetry shows up at birth (one twin is almost always heavier) and often echoes through the first two years of motor development.
Fraternal twins are even more different - they share roughly 50% of their DNA, like any pair of siblings. The fact that they shared a womb tells you almost nothing about how their nervous systems will mature. According to the CDC's Learn the Signs developmental milestone guidance, twins should be evaluated against the standard age-based milestones, not against each other.
Three things drive the day-to-day gap parents notice:
Birth weight and prematurity. About 60% of twins are born before 37 weeks. That gives the first 12-18 months a corrected-age math that doesn't apply to singletons. A twin born at 34 weeks is developmentally closer to a 4-week-old than to a 10-week-old at chronological 10 weeks. NHS twin guidance recommends using corrected age until around the second birthday.
Temperament. One twin is the watcher. The other is the doer. Watchers learn language faster on average; doers hit motor milestones faster on average. Neither is broken. This is the part that doesn't show up on growth charts.
Adult attention math. With two infants and the same number of adults as before, each twin gets less one-on-one mirror time than a singleton would. Language is the milestone most sensitive to this. Twin language gaps usually close once both kids hit nursery or have a structured play environment.
What this looks like at home
My twins are about 4 now. At 14 months one of them was walking confidently and reaching for the bowl on the counter. The other was still army-crawling and perfectly content. Six months later the army-crawler was running. The early walker had moved on to climbing furniture. Same parents, same fridge, same bedtime songs. Completely different timeline.
That gap closed on its own, no intervention. It also reopened on language - the early walker stayed quieter longer. With twins it is rarely one race. It is two parallel tracks where one of them leads in one area, the other leads somewhere else, and the lead changes every few months.
The pattern I missed for a while: I noticed the gap on a Tuesday at the playground when another mom said something. I didn't notice it on a Thursday when both were doing fine.
Decision logic: gap that needs a check vs. gap that doesn't
| Gap | Action |
|---|---|
| 0-2 months difference on any single milestone | Watch, do nothing |
| 2-4 months difference on one milestone domain | Mention at next well-visit |
| 4-6 months on one domain by age 2 | Bring it up specifically, ask about a developmental screen |
| 6+ months OR multiple domains delayed | Pediatric developmental evaluation |
| Either twin missing milestones for chronological age (not just vs. co-twin) | Use standard age-based screen, regardless of co-twin |
Sometimes the answer really is teething plus a cold plus a sibling who is ahead, all at once. Not every gap has a clean explanation. The decision rules above are for when the gap persists across months, not weeks.
For more on the line between a slow start and an actual delay, see our piece on the difference between a late talker and a speech delay. If one twin is much quieter, the signs of speech delay in a toddler guide walks through the language-specific red flags.
What NOT to do
Don't compare them in front of either child. Even at 14 months they are reading your face. The phrase that does damage isn't "Why don't you walk like your brother yet?" - it is the sigh that comes out when you put one back on the floor.
Don't hold the faster twin back to make the slower one feel better. The faster twin needs new challenges or boredom turns into behavior. Two-tiered activities work: same theme, different difficulty. Both kids painting; one with a brush, one with fingers. Both at the park; one on the climber, one in the swing.
Don't accept "they're twins, they should be the same" from anyone, including pediatricians who are not used to multiples. If a doctor compares your twins to each other instead of to age-based milestones, that is the wrong frame.
Don't refuse early intervention because the slower twin "will catch up like her sister did." Early intervention is cheap, low-friction, and reversible. Waiting is none of those things.
When to seek professional help
Bring it up at the next well-child visit if any of these apply:
- One twin has not reached an age-based milestone (per CDC LTSAE checklist), regardless of where the co-twin is
- Persistent 4-6 month gap on the same domain (motor or language) past age 2
- Loss of skills in either twin at any age - this is the one that needs a same-week call, not a wait
- Concern from any caregiver who sees them daily (daycare provider, grandparent)
Most pediatricians will do a basic developmental screen at 9, 18, 24, and 30 months. If you're not asked about milestones at these visits, raise them yourself. According to the American Academy of Pediatrics, parental concern is one of the strongest predictors of an actual developmental issue - more reliable than any single screening tool used alone.
If a screen flags anything, ask for a referral to early intervention. In the US it is called Early Intervention (Part C) and is free under age 3. In Croatia equivalent services run through pediatric early-intervention centers; in Spain, "Atención Temprana" programs are public and regional.
Frequently asked questions
Do identical twins develop identically?
No. Identical (monozygotic) twins share DNA but not in-utero conditions, birth experience, or temperament. Studies of identical twins consistently show 2-4 month gaps on at least one milestone domain in the first two years, with the gap usually closing by age 3.
My twins were born at 32 weeks. When should I stop using corrected age?
Around the second birthday for most preemies, per NHS guidance. Until then, calculate milestones from due date, not birthday. A 32-week twin at chronological 10 months is developmentally closer to 8 months.
One of my twins walked at 10 months, the other still isn't walking at 16 months. Is that okay?
Sixteen months without walking is at the late end of the CDC's normal range (most children walk between 9-15 months). It is worth mentioning at a 15-month or 18-month visit, especially with a co-twin who walked early - the doctor can do a quick motor screen. Most of these resolve on their own.
Should I treat them as individuals or as a pair?
Both, depending on context. Medically: individuals, always. Socially: pair when it helps them, individual when one needs a separate moment. Read more in our piece on how twins develop different personalities from day one.
Is it true that twins can have "private language" that delays speech?
Cryptophasia (twin language) is real but rarer than the term's popularity suggests. Most "twin language" is mutual immature speech that resolves without intervention. It is not by itself a reason for an evaluation, but if combined with limited speech directed at adults by age 2, raise it.
How is autism evaluated differently in twins?
The screening tools are the same (M-CHAT-R at 18 and 24 months), but having one twin diagnosed with autism increases the other twin's risk - roughly 60-90% concordance for identical twins, 0-30% for fraternal. If one twin is being evaluated, ask for the other to be screened in parallel. Our autism vs speech delay guide covers the early signs to watch for.
My pediatrician keeps comparing them and I don't like it. What do I do?
Say it plainly: "I'd like each of them screened against age-based milestones, not against each other." Most pediatricians will adjust immediately. If yours doesn't, the when to seek help for speech delay guide covers what to ask for and what a developmental referral actually looks like.
How KidyGrow helps
KidyGrow tracks each child separately. Same family, different profiles, different timelines, different notes. When you log a milestone for one twin, the app doesn't average it across the pair or auto-fill the other - which sounds obvious but is what most baby apps actually do when you have two children.
By the second week of logging both twins, the daily brief stops saying generic things like "babies this age are starting to walk" and starts saying things like "Twin A walked at 11 months; Twin B is now 14 months and pulling to stand. Pulling-to-stand at 14 months is within the normal range. The 3-month gap is also within normal." The morning question moves from "is the slower one behind" to "this is what the gap actually looks like over time, and here is whether it's narrowing."
The app does not flag a delay just because one twin is behind the other. It uses age-based milestones for screening - the same standard your pediatrician uses. Twin-to-twin comparison is for parents to do (you will anyway); milestone comparison against age norms is what actually matters.
Some weeks the app won't find a useful pattern. Three colds in a row plus a regression in both twins is just illness and chaos. The app is at its best across longer stretches, when the noise averages out and the real trajectory shows up.
For twin families, we recommend logging both children for at least three to five days before reading the early observations - the app needs that time to learn what's normal for each kid individually before it can be useful about gaps between them.
Sources
- American Academy of Pediatrics - Multiple Births: Caring for Twins
- CDC - Learn the Signs. Act Early. Milestone Checklists
- NHS - Twins and multiples: development
- AAP - Developmental Surveillance and Screening
_Educational content. Not medical advice._