I’m not a virus expert but this is what I’ve just learnt!
Zika virus can be passed from a pregnant woman to her developing child and this can cause microcephaly (smaller than normal head). Microcephaly is actually caused by abnormal growth of the brain, not the skull. In development it’s the increase in brain size that makes the skull bigger, so if there is no increase in size the skull remains smaller.
There is a problem with the growth of nerve cells in the cerebral cortex (the largest part of the brain). This problem might be with making new nerve cells or there might be a reason the nerve cells are dying.
Not much is known about how Zika causes this condition and it’s currently being researched.