How to tell my *somewhat abusive* parents I am pregnant at 18 years old?

You could wait until it is too late to have an abortion before telling them. It is a concern that they could turn violent, obviously, especially if such violence would be physical. 

What would happen if your boyfriend accompanied you and filmed "the moment" of breaking the news? They may prefer to behave more kindly, hopefully. Or have a get together with his parents and "make the announcement" then. 

People who are feeling confident and secure and have self respect don't feel a need to call people names, criticise, bully, shout, etc.

Bullies want attention, and my general feelings are that if people allow them to get their way through fear (the ultimate emotional reaction) they are succeeding and will continue. One way to combat verbal bullying is not to ignore them completely as this shows that they are getting to you, but demonstrate that you have heard the remark or whatever with a glance, dismissive wave or brief response ("oh yes", or similar) and THEN ignore them. Why should you care what such a pathetic person thinks of you? (Only weak and needy people feel a need to bully). Alternatively, or in addition, try to use some humour (U.K. spellings!). It need not be mocking humour, but some light-hearted banter can often diffuse situations.

Put on an imaginary suit of golden armour, and visualise the unkind remarks (or those you perceive to be unkind - be very careful not to imagine a slight when none is intended) as arrows, harmlessly bouncing off your armour and falling to the ground.

If the abuse is physical, you could join a martial arts or self defence class. Or even check out some moves on Youtube. Learn to defend yourself without retaliating.

Good Luck!

You have to decide what your goal is in telling your family.  If you want congratulations and offers of support -- and you don't think that is likely -- then don't tell them at this point.  If you just feel you want to make them aware then you have to be prepared for negative reactions.  

There is no reason you should be in direct physical proximity to anyone who you fear might attack you physically.  You can call, text, email, etc.  You also are not required to answer phone calls, replay to texts, or read emails from people who are mean, manipulative, or threatening.   Just because you provide people with some information doesn't mean you have to engage in additional communication with those people.  

You just tell them, and there will always be some who bring up the abortion option, becouse it is an option. Don't get offended by it, you have your plan and that  is not it, end of story. So you left at 17 but they still financially support you, at least some? That is odd but whatever works. That might come to an end now, old enough to have a baby and crap.