Whiteorchid, no charge I am a fitness GEEK and love my job! If you've been exercising before and throughout pregnancy you can keep doing a plank for a while if you're comfortable.
I would start to watch out for your body position being compromised by your bump (am sure you already know you need to be in a solid straight line from shoulders through hips to knees). Also be careful of holding a position like the plank, which can cause your blood pressure to go up - so I would go for holding it for far less time, dropping down on to your knees and then pushing up again. No holding your breath! Also, because of the weight at the front you'll find your back is working much harder.
You could start doing an incline plank with straight arms - the angle will just take a little bit of strain off, but will still be good work.
As you go on and your bump inevitably gets bigger, you will find it harder and harder to do the plank and you will get uncomfortable. As soon as you feel even slightly uncomfortable, stop and change your routine. This isn't the time to be brave and push on!!
Don't forget the good old press up (knees down) which uses a plank style position and will also work your chest/upper back/upper arms as well as your abs. Try using the swiss ball as much as possible too.
Also don't underestimate how much your abs are working all the time and especially when you're exercising. As your bump gets bigger, try standing in front of the mirror and looking at the difference in the shape and 'lift' () of your bump when you tighten/brace your abs! If you do this regularly, then it's actually a really great workout as, when you think about it logically, it's weight lifting for your abdominals!
I felt v. odd running in my first pregnancy, so didn't run a step. This time it feels totally and utterly normal - so am enjoying it while I can!
Please don't overdo it anyone, I love fitness and love my job but it's easy to get carried away esp when you feel good in the 2nd trimester. Exercise, eat well, eat in reasonable quantity and enjoy being pregnant.
I'm pretty evangelical about exercising in pregnancy, I have spent a lot of time poring over the Royal College of Obs/Gynae research on the subject, I badger the midwives for info and they've helped me loads, but it's also important to remember that eating CAKE when you're pregnant is good too!
(shuffles off to be fitness geek in corner on her own.)