Eat healthy. Low carbs and sugar. High protein. I have lost weight this pregnancy by doing that. I have scrambled eggs, whole wheat toast and Greek yogurt for breakfast and lunch and dinner I have meat and veggies. If I have rice or something my serving is no bigger than the palm of my hand. If you are craving something go ahead and have it, but watch the portion size. And just try to walk a couple times a week.
Excersize is totally an option unless your doctor advises you otherwise. I'm an overweight ftm and I've started going to the gym 3 times a week. I'm careful not too push too hard for too long and keep an eye on my heart rate. I'm not there to lose weight but to strengthen my heart, joints and back and to increase my endurance.
I am also 25, overweight and pregnant with my third! Just try to eat healthy, the less sugar and starches you have the better. It is the third so I am sure I will gain a little more but by eating right I was able to stay under the 10 lb gain with my other two so am trying to watch it again. Also try to work in some exercise a few times a week, I know it is hard with 2 little ones already but I feel better when I work it into my day. Congrats and good luck!