What is the next bra size after 40d?

The next bra size after 40D would typically be 40DD (or 40E, depending on the brand). Bra sizes progress in increments of one cup size, so moving from D to DD (or E) is the next step. Additionally, you could also consider going up in band size to 42 while adjusting the cup size down to maintain a similar fit, resulting in a 42C.