Is a10cm increase in head circumference in 16weeks something to be alarmed about? my twin boys heads have gone from 32cm(at birth) to 42cm in 16weeks

Yes: Head circumference is expected to inrease 1/2 in (1.77 cm) per month in the first 4 months. Rapid increase in head size could be a sign of hydrocephalus. Have them evaluated.

Yes & no: This deserves watching. If the kids were born at ~36wks, this marks a move from the 25% to the 75% head size & if the soft spot seems to bulge when sitting up i would be concerned about pressure build up. Yet some twins do experience a bit of catch up growth after birth & this could be ok. If it correlates with improvements in linear growth (length) of a similar amount i would worry less.