I always gain around 40-45lbs, always. And that's usually without going to term as well (36, 35, 38 weeks). This is DC4, and I'm predictably on track for that at almost 30 weeks having already gained 32lbs. And it shows, and I don't just mean on my bump.. even clothes that go nowhere near the bump don't fit any more and my double chin is back, joy.
I feel like I have an excuse in that I'm normally very active (40-50 miles of walking a week with a pushchair plus a very outdoorsy life) and have an appetite to match that and fuel it. However I get SPD really early on, so my walking drops quite quickly, but my appetite remains. By the time I hit 20 weeks I can barely manage a few miles spread throughout the day with school runs, shopping etc, and at this point now at nearly 30 weeks it is a struggle to do 100 yards. With a crutch. I'm already on the mobility scooters in malls.
I am eating less, but nowhere near the sort of portion sizes associated with my dire levels of activity, because basically I am depressed about my mobility issues and stuck in the house all day most days with nothing to do so I just eat instead. My willpower is at an all time low.
Once baby is born I only lose a stone of it too. I leave it a few months post birth - even though I EBF I never lose anything through that - then start the weight loss plan. Takes me around a year of lots of walking and calorie counting to get back to a healthy BMI again. So I know I do have the willpower to eat properly, it's just it flies out the window in pregnancy.
I try hard not to beat myself up over it but I am so 
of people who can remain active and/or seem to just not gain very much. And those for whom the weight just "falls off". It's just not in my genetics. Nor is remaining stretchmark free..