When one child is diagnosed with autism, many parents look at the baby or toddler next to them and wonder. That question is reasonable, and asking it early is helpful. This guide explains what research says about autism in families, what extra monitoring usually looks like, and how to arrange screening and early support in Florida. It also covers how to keep the worry from taking over the time you have with your younger child.
How likely is autism in a younger sibling?
A large recent estimate comes from the Baby Siblings Research Consortium, published in Pediatrics in 2024. Researchers followed 1,605 infants who had an older sibling with autism. The American Academy of Pediatrics summarized the findings:
- About 20.2% of the younger siblings were diagnosed with autism, roughly 1 in 5.
- When a baby had more than one autistic older sibling, the rate rose to 36.9%.
- Younger brothers were diagnosed more often than younger sisters (25.3% vs 13.1%).
- Younger siblings of autistic girls were significantly more likely to be autistic than younger siblings of autistic boys.
Put the other way, about 4 in 5 younger siblings in the study were not diagnosed with autism. The authors' conclusion "emphasizes the need for close developmental surveillance of new infants in those families."
These are group averages from research families, not a prediction for your child. Your pediatrician or a genetic counselor can talk with you about your own family.
What does extra monitoring look like?
The CDC says the American Academy of Pediatrics recommends developmental screening for all children at 9, 18 and 30 months, and that all children should be screened specifically for autism at 18 and 24 months. The CDC adds that "additional screening might be needed if a child is at high risk for ASD (e.g., having a sibling with ASD)." In practice, that can mean:
- Telling the pediatrician at every visit that an older sibling is autistic, so it is in the chart.
- Asking about developmental and autism screening at visits beyond the standard ones.
- Using the CDC's milestone checklists between visits and bringing them to appointments.
- Asking for a referral for evaluation as soon as there is a concern, rather than waiting for the next scheduled screen.
A simple log helps. Keep one page per child, with the date, what you noticed, and in which setting (home, daycare, a family visit). Note the good moments too: the first point, a shared laugh, a new word. When you meet with the pediatrician, the log turns a general worry into specific examples, and it helps the doctor see change over time. If your older child's evaluator or BCBA knows your family, they may also notice things worth mentioning, but the younger child's own doctor should lead any screening decisions.
The M-CHAT-R/F, a common toddler autism screen, is valid from 16 to 30 months. Before that age, pediatricians rely on developmental surveillance and general developmental screening. See the M-CHAT-R/F guide.
Which early signs should I watch for?
The CDC lists signs such as not responding to name by 9 months, using few or no gestures by 12 months, and not pointing to show something interesting by 18 months, as well as repetitive behaviors and unusual reactions to sounds, textures or tastes. Many babies do some of these things sometimes, and one sign alone does not mean autism. Look for patterns over weeks, and write them down with dates.
A practical tip for families who already know autism: your younger child may look different from your older child. Siblings often show different signs, at different ages, with different strengths. Try not to compare them one-to-one. See early signs of autism by age.
How do I arrange screening in Florida?
Through your pediatrician
At your younger child's next visit, say: "Our older child is autistic. I understand younger siblings have a higher chance of autism. Can we plan regular developmental and autism screening, and can you refer us for an evaluation as soon as there is a concern?" Our pediatrician script has more wording.
Under federal preventive rules, autism screening at 18 and 24 months is generally covered at no cost in network for non-grandfathered plans. Screening at other ages may be billed differently. See is autism screening covered by insurance.
Through Early Steps (under 3)
Florida's Early Steps program accepts referrals from anyone, including parents, and does not require a doctor's referral or a diagnosis. You can refer online through the Early Steps website or call 1-800-218-0001. The local program must complete the evaluation and initial family service plan within 45 calendar days of the referral. To qualify without an autism diagnosis, a child generally needs a measured developmental delay (or qualifying informed clinical opinion) under Early Steps' criteria, so a referral is not a promise of services. If your child does qualify, Early Steps does not charge families out-of-pocket fees for its Part C services. See how to refer to Early Steps.
Through your school district (age 3 and older)
From age 3, a parent can ask the school district in writing for an evaluation. Under Rule 6A-6.0331, the district must ask for consent or explain in writing why it refuses within 30 days.
If my younger child is diagnosed, does that make things easier?
In some ways, yes. You already know the systems, and you may already have a provider your family trusts. A few things to know:
- Each child needs their own diagnostic evaluation and report. A sibling's diagnosis does not carry over.
- Each child needs their own insurance authorization for ABA, based on their own assessment and treatment plan.
- Younger children diagnosed early may benefit from early, play-based support. The guide to ABA for toddlers explains what that can look like.
How do I keep worry from taking over?
Watching a baby for signs can turn every missed wave into a crisis. A few approaches help:
- Set check-in times. Fill out a milestone checklist at set ages instead of watching all day.
- Let the professionals carry some of it. Once a plan for regular screening is in place, trust the plan.
- Enjoy your child as they are. Play, songs and cuddles are good for every baby's development.
- Include your older child. Autistic siblings and baby siblings can build real bonds. For ideas, see sibling support.
- Get support for yourself. See parents' feelings after an autism diagnosis.
How Hope Center can help
If you have a concern about a younger sibling, Hope Center for Behavior Change offers autism diagnostic evaluations through our clinical director with a supervising licensed psychologist. If a child is diagnosed, we provide play-based, assent-based ABA in the center, at home, at daycare or in the community, with weekly parent training, and we have no waitlist. We can check benefits for each child for free. Call 561-337-8865 or request a free consultation.