mvc is mean corpuscular volume
which is one of the parameters of blood cells,
common cause of increased mvc is megaloblastic anemia
which occurs due to deficiency of vitamin B12
and folic acid
these are used in formation of cells, so though your previous levels were normal and you are taking good diet, in pregnancy the need will increase as these are utilized by baby, so that might have led to deficiency in you, by taking supplements mvc levels can come to normal.