First, MRI's are proven to promote and spread the growth of cancer cells. Save it for a life and death situation.
Second, seriously?! My kids ALWAYS used their right hands since birth, grabbing at things, picking up things, etc. This was NEVER a cause for concern. We also come from a family of people, both sides, who are ambidexrious... even with that family history, this was never a cause for concern. At 15 months, kids are going to whatever it takes to get their hands on what they want... I would get a second opinion from another pediatrician if you're that worried about it, but I honestly think you shouldn't waste another second worrying about it.
Being delayed in social and communications skills isn't necessarily something to worry about either. It's about age and maturity levels. My baby brother didn't talk until he was 6 because he had 2 older siblings to communitcate for him... he graduated high school 3 years early and is borderline genius now.
If it were my child, I wouldn't think twice about it, and I certainly wouldn't spend time/money/my childs well being on an MRI for something so trivial. Unless there is some other cause for concern (abnormal blood test, previous MRI/CT scan that showed abnormalities, or a family history of genetic disorders) I would let it ride and see what the outcome is. Most kid switch up their hands until they are actually writing at school age.
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