Early intervention. I would recommend physical therapy and if needed speech therapy to help your baby develop as close to normal for age as possible. I am not sure that medicine is what is needed, since I do not know the medical history of your baby, but early intervention involving physical therapy, speech therapy and occupational therapy could help.
Pediatrician. You need to talk to your pediatrician to determine why there is a developmental delay. Do not give your child anything without discussing it first.
You've gotten good. Advice. Rarely is there a medication that addresses the underlying genetic/medical disorders that cause developmental delays. To maximize her developmental potential, enroll her in your state's early intervention program for needed therapies. At the same time, ask your pediatrician for a referral to a developmental/behavioral pediatrician (found on sdbp.Org) to look for the etiology (cause).