Congratulations! I will be 20 weeks along tomorrow, and I have been McDougalling for nearly 3 years now. This is also my first pregnancy.
I have eaten mainly the MWL plan for the last 3 years, and have continued into this pregnancy. I do eat a bit more fruit than recommended most day, and I eat puffed rice, rice cakes, and corn tortillas if i am needing calories and can't fit in more bulk. I don't eat nuts and seeds except for 1 Tbsp of flaxseed most days.
My basic diet consists of lots of raw/cooked veggies, starches (gluten free for me....mainly sweet potatoes, potatoes, quinoa, brown rice, and sometimes millet or gluten free oats), fruit, and very very occasional beans.
I supplement with B12 each day, and also take my ground flaxseed nearly every day to ensure i get my omega 3's. I have also made a bit of a conscious effort to increase my protein a bit during pregnancy. I don't eat beans (digestive issues), so i do this by trying to eat quinoa daily, eating lots of potatoes and green veggies, and minimizing the fruit a bit more. My midwife approved my diet as excellent, though low in protein sources (because she was counting meat, dairy, beans etc. instead of looking at my overall protein grams per day).
Dr. McDougall and Jeff recommend the same healthy, whole foods diet before, during, and after pregnancy. You might find you eat more sometimes, based on appetite. This will enable you to meet all your needs and your baby's nutritional needs. Just be sure to eat real, whole, plant foods so you get all the nutrients you need.
In the first trimester, I felt very nauseous, and most of my usual veggies sounded horrid to me. I did my very best to include as many veggies as possible by trying different varieties of lettuce, eating any veggie that didn't sound nauseating to me, and adding veggies into things like marinara sauce (i tolerated marinara sauce and brown rice pasta for a few weeks). I usually eat very simply (ie. plain starches, plain raw veggies etc.), but there were several weeks where I had to cook more elaborate meals for myself to make things that sounded at all appealing. I did my best, and managed to get through it alright. Now, i'm mostly back to normal and enjoy a variety of veggies, starches, and fruit most days. I have never had a day of constipation
So, basically, eat whole plant foods to the satisfaction of your appetite. If you're worried, you can check your diet on cron-o-meter to make sure you're meeting your requirements, and then adjust as needed. Also, take B12 every day, and some ground flaxseed if you're not meeting your omega 3 requirements through your diet (you need about 1.1g per day).