Microcephaly means exactly what it says: Micro (small) cephaly (head). When the brain doesn't grow, the skull won't grow. Thus, an individual will have a smaller brain and smaller head. Microcephaly is diagnosed when the head circumference is below -2 standard deviations. Severe microcephaly is when the head circumference is -3 standard deviations. Samantha's head circumference is so small, it isn't even on the chart.
There are many causes for microcephaly. Among them are mother's alcohol consumption during pregnancy, infections during pregnancy, microcephaly secondary to other genetic conditions and disorders, and in rare cases when the mother and father both carry a recessive gene, pass it down to their child, and it then becomes dominant within the child. This last example is Primary Autosomal Recessive Microcephaly, or Microcephaly Vera. This was Samantha's first diagnosis. Some genes have been identified, but research is still being done.
Even rarer is Microcephaly with Simplified Gyral Patterns Type 1 (MSG1). This means that the actual pattern in the brain of the gyrai is simplified, not as complex, not as many folds in the brain. In June of 2011, a paper was published through University of California, San Francisco that shed some light on this new condition. The article suggested that MSG as a subset of Primary Microcephaly, or Microcephaly Vera. It's still too early to know the cause of MSG as there are so few cases (about 17 with Samantha's type), but for more detailed updates on Samantha and a list of resources and therapies, you can follow her own blog ~ Lil' Samsquatch.