For instance, babies who suffer from mental retardation due to fragile x are often born with things like flat feet, strabismus, double joints, low muscle tone. Are those, and similar characteristics the characteristics that set fragile x apart? I know the ate lots of characteristics a fragile x baby is born with but that's all j could think of. Also is "stimming" more common in people with fragile x vs. someone who became intellectually impaired for other reasons?
The only kind of drug I can think of that causes an obvious mental disability is fetal alcohol syndrome. But the physical characteristics are pretty obviously different from fragile X.
Stimming is honestly a big red flag for autism. All spectrum of autism seem to have that in common. It might be rocking back and forth or other similar motions, but that repetitive body movement is something I know they look for when trying to diagnose autism (again any spectrum). I've only ever met one child with fragile X and he was diagnosed under the umbrella of autism too. So my experience with fragile X is pretty much limited to textbooks.
I do know of one child I grew up with who, although she had a reading disability, she had normal intelligence and no obvious signs of autism. She used to flap her arms whenever there was something exciting or big happening around her. When she was about 11 or so a doctor realized she was legally blind and found some corrective glasses; her stimming stopped. For her, Stimming was a way to unleash the tension from all the sensory input that her body couldn't make sense of. So I know from at least one case, stimming isn't necessarily caused by a mental disability or autism.
I cannot speak to what other conditions may or may not mimic the symptoms of fragile x, but what sets fragile x syndrome apart from other mental and developmental delays/autism spectrum disorders is that it has been mapped to a specific gene location on the human chromosome (genotype), and there is some understanding how the expansion manifests itself (phenotype).
Although many other causes of developmental/autism spectrum disorders have yet to be identified, fragile x is specifically an expansion of genetic material on the X chromosome. Though it's possible that many other chromosomal variations and/or drug exposure in-utero can cause similar symptoms after birth, fragile x is an inherited disorder, and its presence cannot be impacted by a mother's exposure to drugs or medications during pregnancy.
At this time, there is no "cure" for fragile x; identifying the cause of mental disability/autism as fragile x or not helps physicians and caregivers identify the best course of management. Hope that helps.