I agree with the need to make sure you get plenty of vitamin B-12, which comes only from animal products. Also make sure to eat nutrient dense foods, not just anything because it's not from an animal.
In addition to everything else that's been mentioned, you should be sure to have your thyroid function tested to make sure there's enough thyroid hormones for, both, you and your fetus. Thyroid hormones are essential for the growth and development of a fetus and they're also essential to your own metabolism. Ask your doctor to test Free T4, Free T3 and TSH. Just having the hormone levels "in range" isn't good enough... Free T4 should be, at least, in the middle of its range and Free T3 should be in the upper half to upper third of its range...
Hey there. I have a very overweight friend. Guess what happened when she got pregnant? She lost a bit of weight. :>)) That's really common. Because you burn more calories when pregnant. You don't need to eat any more in terms of quantity or calories --- but have a pregnancy going which burns more. Make sure you get lots of protein when vegan/vegetarian and you should be good. Keep exercising and yes, don't diet. Just don't increase how much you eat. And after you have the baby, focus on cutting calories then while increasing exercise and it could be the start of a whole new you after baby. :>) Congrats on the pregnancy!!
Not a pregnant female (or any other kind) but the mere fact you eat a vegan diet has nothing to do with whether you're eating a healthy diet or one suited to losing weight. You also say you work out, but don't say how much, so it might not have been enough for you to speed up your metabolism. Some people are not suited to vegan or vegetarian diets, and some are -- you might be one of them and you might not be, but most vegans eat according to ideology, not health. Vegans still have to balance their diets so they get sufficient nutrients and don't overdose on simple carbs and unhealthy foods. So make sure you still get ample protein and that you still eat your veggies and doing what is healthiest and best for your metabolism. Make sure you get tested for B12 and methionine, essential for health for you and your child but hard to get on vegetarian diets if you don't really know what you're doing. I have to tell you, when I managed health foods stores, the least healthy eaters were the vegans -- they were so convinced just not eating animal made them so righteous that they'd eat anything, including scarfing down enormous amounts of hydrogenated vegetable oils that made up their desserts. So make sure you stick to healthy eating even if you're vegetarian or vegan.