Is it safe to sleep and or lay on my back at 18 weeks pregnant? can i also sleep on my right side?

Yes: You don't have to worry about positional sleep habits until later in the pregnancy. Right now the uterus isn't big enough to cause compression. Later in the pregnancy, say, after 28 weeks or so, best to try to lie on your right side. Having said that, many mommies have accidentally fallen asleep on their backs late in pregnancy with no harm to their babies and only some swelling in their legs.