At 18 weeks pregnant, I wouldn't be looking to change anything up at all. Stay right where you are and make changes as the pregnancy progresses that enable you to have more energy and get more comfortable. The trend will likely be scaling back and down, not adding nor increasing, apart from weight and size, both of which are necessary! Your sense of balance, your stability, your centre of gravity are all going to be constantly changing over the next 7 months. Hoisting a heavy barbell for leg day? I don't think you need to be worrying about that, quite honestly. Your focus right now is best dedicated to health for you and baby, energy, sleep, nutrition and keeping your fitness level for as long as you can, but the pregnancy will dominate over fitness and will dictate terms, make no mistake! Let it happen: it is supposed to. You have the rest of your life after giving birth to add as many leg days as you wish!
Clare