I am from Nigeria, and I have UK visa to visit for 6 months. I didn't use it to travel because I am pregnant and due for delivery. My doctor says I can't travel. Will I be able to get another visa when next I want to travel again? I don't know if this will affect my chance of getting another visa.
Yes you should be able to get another UK visa without any issues.
Of course you must make sure that you don't violate any other visa conditions. As mentioned by @DumbCoder, UK specifically stops pregnant women, becuse they can give birth in UK and then demand UK residency because of the child