they'll help with strength. They're the only exercise I continue when pregnant, and after baby comes out belly is back to flat. I blame planks. But, unless you're in a calorie deficit whether by diet or working out, you won't lose much in the way of inches.....

I do planks but not to lose inches my core has gotten insanely strong in the last two months from the addition of proper planks. I do 2 minute forearm planks every other day. Started out only being able to hold it for 20 seconds. Currently increasing from 2 to 3 minutes and held one today for 2 minutes 15 seconds. In daily life, I've noticed this strength mostly at night. It might sound weird, but I toss and turn a lot at night. And I sit up a lot at night to adjust my blankets (I have a wack joint condition). Anyway, I used to have to push myself up into sitting position using my arms to push my upper body up or roll to the side and press up. Now I just sit up Straight from laying flat on my back without pressing up with my arms, like something from the exorcist or something- haha. I didn't start doing this consciously, it just evolved over time. Pretty cool in any case although it probably looks freaky.
I also do about 5 minutes of core pilates moves 3x a week.