What weren't you told, and what did what wasn't told have to do with your baby's birth defects? How did you being underweight affect anything? There's not nearly enough information here for anyone to evaluate whether or not you have a case against your doctor.
Before asking a lawyer for advice, first find another doctor who will tell you that the first doctor did something or didn't do something that was below the standard of care for those kinds of doctors, and that error or omission caused harm.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
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