The right time to be fitted for a maternity bra varies from woman to woman and there is no right time for everyone. The breast begins to prepare for lactation quite early in pregnancy and some women will outgrow their usual bra size earlier than others. Generally speaking, most of the changes to the breast have occurred by around four months (16 weeks gestation) and this is a good time to be fitted, if you haven’t needed to earlier.
A correctly fitted bra gives you comfort and support, so it is a good idea to be professionally fitted. This is a free service. It is not necessary to buy a bra you will grow into – indeed too big can be as bad as too small!