Thank you for your query.
Decreased head circumference is called microcephaly
However, in your case, the difference between "normal " and your value is very less.
There are various causes of microcephaly such as congenital, certain infections, chromosomal disorders etc.
If the anomaly scan is normal, you need not worry about anything.
A repeat scan as recommended will clarify the picture, at present there is nothing else you can do, apart from a nutritious diet, regular ante natal care and supplements as advised.