In one of our Studycat family stories, we covered how Emiliano was 2 when his parents first noticed he was behind on his words. Now he’s 6, in speech therapy for English while learning Spanish at the same time, and both are improving together. His story is a common one, and it points at a real, persistent myth: that bilingualism itself causes developmental delay. Research measuring a bilingual child’s vocabulary across both languages combined, rather than judging each language on its own against a monolingual yardstick, tells a very different story. A study in the Journal of Child Language found that bilingual children reach early language milestones, first words, first phrases, on the same timetable as monolingual children.
Key takeaways
- Bilingual children reach major early milestones, first words and first multiword phrases, on the same general timeline as monolingual children
- Bilingual children often have smaller vocabularies in each individual language, but their total vocabulary across both languages is comparable to, or larger than, monolingual peers
- CDC milestone checklists remain accurate for bilingual children when a parent counts words and skills across both languages combined
- A real speech delay and typical bilingual development can look similar early on, which is why tracking total vocabulary matters before assuming a problem exists
Where the “bilingualism causes delay” myth comes from
The myth has a kernel of truth in it, which is part of why it spreads. Bilingual children often do have smaller vocabularies in each individual language compared to monolingual peers, simply because their exposure to each language is divided rather than doubled. A parent comparing their bilingual child’s English vocabulary directly against a monolingual English-speaking peer’s checklist can understandably see a smaller number and worry.
The problem is the comparison, not the child. Measuring one of a bilingual child’s two languages against a monolingual benchmark was never a fair test to begin with.
What the research says when you count both languages
When researchers count a bilingual child’s total vocabulary, words known across both languages combined, the picture changes substantially. A UK study comparing bilingual and monolingual toddlers found their combined vocabulary size was comparable to, or larger than, monolingual children’s single-language vocabulary. Researchers Laura-Ann Petitto and Ioulia Kovelman found that bilingual children hit the classic early milestones, first word around their first birthday, the 50-word mark, and combining words before their second birthday, on the same general timetable as monolingual children, in both of their languages.
That’s the piece missing from most milestone conversations. A bilingual child isn’t behind. They’re often building two vocabularies in parallel, at a combined pace that matches or exceeds a single-language peer.
Using CDC milestones with a bilingual child
The CDC’s (USA Centers for Disease Control and Prevention) “Learn the Signs Act Early” program provides free, well-researched milestone checklists from 2 months to 5 years, covering how children play, learn, speak, act, and move. These checklists remain a genuinely useful tool for bilingual families. The adjustment is in how a parent applies them: when checking language milestones specifically, count a skill as met if a child demonstrates it in either language, and total the vocabulary across both rather than scoring each language separately against the full checklist.
This isn’t about lowering the bar. It’s about applying the same checklist accurately to a child who is developing in two languages instead of one.
Try it at home:
When you use a milestone checklist, keep a simple running list of words your child uses in either language, don’t worry about which one. If the combined list is growing steadily, that’s the real signal to watch, not how it compares language-by-language to a monolingual peer.
Why Emiliano’s story isn’t unusual
Sonia, Emiliano’s mom, isn’t fluent in Spanish herself. Neither is her husband. Emiliano is building it through school, home exposure, and five-minute stretches during the morning school run, with no dedicated study time or set schedule. That kind of setup, a child learning a language a parent isn’t fluent in, is a common routes into bilingualism, and where the situation where the “bilingualism causes delay” myth tends to hit hardest, because a parent already unsure of their own role can read a smaller single-language vocabulary as confirmation something is wrong.
The bigger point for any bilingual family is that “behind” and “developing differently” aren’t automatically the same thing. Milestone tracking done fairly, across both languages, is what tells the two apart.
When to actually be concerned
None of this means bilingualism is a reason to dismiss every worry. A real speech or language delay can occur in bilingual children just as it can in monolingual ones, and it’s worth flagging to a specialist like Sonia did, if a child isn’t hitting the combined-vocabulary milestones in either language, not just one.
Frequently Asked Questions
Does learning two languages delay a child’s speech?
No. Research shows bilingual children reach early language milestones, like first words and first phrases, on the same general timetable as monolingual children when both languages are counted.
Why does my bilingual child seem to know fewer words in English than other kids their age?
Because exposure to each language is divided, a bilingual child’s vocabulary in a single language is often smaller than a monolingual peer’s. Their combined vocabulary across both languages tells a more accurate story. Our guide on suitable ages for Studycat has more on what to expect at each stage.
Should I still use standard milestone checklists for my bilingual child?
Yes, with one adjustment. Count a milestone as met if your child shows it in either language, and total vocabulary across both rather than judging each language on its own.
Scientific References & Further Reading
- Cambridge University Press. Bilingual children reach early language milestones at the same age as monolingual peers. Journal of Child Language. https://www.cambridge.org/core/journals/journal-of-child-language/article/bilingual-children-reach-early-language-milestones-at-the-same-age-as-monolingual-peers/0BFAB689DC70E5A2A5220976E1939948
- Kovelman, I., Baker, S., & Petitto, L. A. (2008). Age of bilingual language exposure as a new window into bilingual reading development. Bilingualism: Language and Cognition, 11(2), 203-223.
- Centers for Disease Control and Prevention. Learn the Signs. Act Early. https://www.cdc.gov/act-early/milestones/index.html
- Siow, S., Gillen, N., Lepadatu, I., & Plunkett, K. (2023). Double it up: Vocabulary size comparisons between UK bilingual and monolingual toddlers. Infancy, 28. https://doi.org/10.1111/infa.12562
- Core, C., Hoff, E., Rumiche, R., & Señor, M. (2013). Total and conceptual vocabulary in Spanish-English bilinguals from 22 to 30 months: Implications for assessment. Journal of Speech, Language, and Hearing Research, 56(5), 1637-1649. https://doi.org/10.1044/1092-4388(2013/11-0044)
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