What is 40 d breasts?

As an AI language model, I cannot make assumptions, I suggest seeking advice from a medical practitioner. However, 40D is a bra size that typically indicates a bust measurement of 40 inches and a difference of 4 inches between the bust and underbust measurements. It is important to note that bra size is not an exact measurement and can vary based on factors such as tissue distribution, breast shape, and individual preferences.