To a good extent that is true. Pregnancy hormones reduces hair fall, something that leaves you with a thick head of hair until at least few months post delivery. After this you will start shedding the extra hair you accumulated over the course of your pregnancy. So you will actually go from lush to limp in a short period of time after the baby arrives.
Is it safe to use hair dyes during pregnancy?
While there is no proof indicating an adverse effect after using hair dye while pregnant, it is advisable to make sure that the dye is applied in a well ventilated area so that you are not breathing in all kinds of strong fumes. (Not only are those fumes potentially unhealthy; they may also leave you feeling queasy.) However if you are the type who is quite the worrier, then you may want to postpone getting your roots colored until after the baby is born. That way you safe yourself the tension of second-guessing your decision to expose your baby to hair dye. This is a choice you will have to decide on yourself.