I would say this is a good opportunity to practice being a "proper" adult!
At some point in growing up, you learn that other people's feelings are not your responsibility. You take ownership of your choices, and allow other people to feel how they want to feel about them, without letting it affect you too much.
That's part of maturity, accepting that other people don't have to keep you happy all the time, and you don't have to keep them happy all the time.
I would say that if you have made your choice, there's nothing to be gained by waiting to tell your parents. The sooner you tell them, the longer they have to get their heads around it.
If they are upset and saying hurtful things, you could try telling them something like, "I understand why you are upset, and I know it might take a while for you to accept my decision. I'm happy to talk with you about how I plan to afford and care for my baby, but I would like to keep it respectful. If the discussion becomes about criticising me or trying to change my mind, I will leave. If it's about figuring out the best future for me AND my baby, I'm happy to listen any time."
I'm sure you already have had thoughts around this, but it might be best to go in with a solid idea of (as above) you are going to pay for and look after your baby. That will reassure them.