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Bilingual children are not automatically delayed in speech development; if a child produces fewer words in one language, the correct scientific measure is their combined vocabulary across both languages — not a one-sided comparison to monolingual peers based on a single language.

Quick answer

Bilingualism is not a root cause of speech delay. Between about 18 and 30 months, a child may say fewer words in each language taken separately, but their combined expressive vocabulary across both languages is usually normal. Mixing the two languages at this age is a completely normal cognitive process. Only if you see a clear weakness in both languages, or in social interaction, does a clinical evaluation become necessary.

What does the science say?

Developmental psychology studies show that infants can tell apart the sound patterns of more than one language within their first months of life, and their brains are biologically ready to learn multiple languages at once without experiencing cognitive confusion. When parents worry about speech delay, they're often judging a child's ability in just one language context — say, Persian at home — while a meaningful share of that child's vocabulary has actually developed in the other language. Speech-language pathologists stress that the myth linking bilingualism to speech delay needs to be corrected, and that evaluations should be based on a child's total communicative ability.

• Infants and toddlers can learn more than one language without confusion or inherent delay, as long as they receive rich language input.
• When you measure vocabulary in each language separately, a child may appear to have a smaller vocabulary than monolingual peers — but combined vocabulary is the more scientifically valid measure.
• Mixing languages in young children is not a sign of a disorder; it's a natural cognitive strategy to fill a gap in one language's vocabulary.
• Clinicians recommend evaluating language skills comprehensively, taking both language systems into account.

Quick practical tip

Keep your language routine consistent at home. Setting aside dedicated time for each language — for example, everyday interaction in Persian plus 15–20 minutes of play or age-appropriate cartoons on the Lingome watch hub with active parent interaction — helps a great deal. If your child uses a word from the other language, simply say it back in the right language without direct correction. Track language growth by recording new combined vocabulary across both languages.

Common mistake: Dropping one language in hopes the child will start talking sooner, and constantly comparing them to monolingual children. — Evidence-based approach: Stay consistent with both languages, measure combined vocabulary, and see a specialist only if there's a delay in both languages at the same time.