Just a Belly Workout

For your core, you want to focus on your transverse abdominis. That's the coreset muscle that really holds everything in and flat...and it gets totally destroyed during pregnancy.

Easiest things you can do right now:
-Deep breathing. At the end of the breath, pull your belly button in towards your spine as much as you can. Hold for 5. Repeat. Do 3 sets of 20.
-On your hands and knees, suck your belly in and release. 3 sets of 20.

Once you have the baby...same things, plus the planks the other poster mentioned. I did the deep breathing every time I would get up to feed the baby. When all you're doing is sitting there so baby can nurse or have a bottle, it's easy to get these done.

PS: Why would you have to wait until "getting cleared" at 6 weeks for your doctor. I'll never understand this... Unless you had a C section or complications...you can really start working out whenever you feel up to it.
With both of my kids (and 1 was a section), I started working out again the week I came home from the hospital. I'm a fitness person, and worked out up until delivery with both of my kids. Sitting there idle just drove me nuts.

Planks.

Do lots and lots of planks. HOld for 10 to 20 secs each time. Once you get stronger you can move your arms in a clockwise fashion (right hand to 3pm, 6pm, etc) and then do the left arm. Eventually add in the legs in the same pattern.

Good luck!

OMG I can't believe someone suggested that everyone should heal as quickly as she did after her pregnancy. What a dangerous assumption that every woman can start working out as soon she feels able. Goody for some people who can work out up till the minute they deliver. I guess they've had perfect pregnancies not riddle w/medical problems, abnormal births or bed rest. Hallelujah for those women who never have to deal w/high risk pregnancies b/c Heaven forbid our lives be unbalanced in the world of pregnancies.

Please follow doctor's orders. If the doc tells you to wait then do it. Some people heal quicker than others. Your doc might see a bruised lining, thin membranes or a slight abnormality in the shape of your hips or something, and so he/she gives a certain timeframe to be active again so your body heals well enough so you won't tear or bleed and can carry another baby in the future.

I am gonig to ditto exactly what H&H said - planks - side planks, front planks, one leg up planks, elevated planks, alternating side planks, one legged planks and knee lto chest planks.
They tighten the whole core, are 'low impact' and can typically be done with your baby since you are so close to the floor. I always put my kiddos underneath my face (or near my face so I didn't fall on them!) and make funny sounds and eye contact while doing these work outs. with #2 my older child really liked playing "bridge" with mommy:)
There is also a great book called Yoga Baby and it leads you through basic yoga moves that incorporate your child and help tone and stretch where you need it.
Congrats and good luck!

Yes, definitely wait to be cleared by your doc. I too am very thin. I ate like a trucker while breastfeeding and only did it for 5 months. I never put an ounce back on after stopping and still eat quite a bit. I'm in the double digits and definitely don't need to lose any more. However, I will tell you, after baby #2, I need to do MORE than just tummy exercises. My butt is jiggly and hangs down and I am not strong enough to carry my baby in his carseat. So, you might want to think about working your whole body, not just your tummy. Be sure you eat plenty so you don't get too thin. I will also warn you, be sure you don't have diastasis recti. Because if you do normal ab exercises, you can make it even worse. I can fit a couple of fingers between my abdominal gap. I was doing reverse crunches about 6 weeks post partum and I can't help but think I made it worse. The mama that suggested working the transverse muscles was spot on. Check out maternalfitness.com. That's where I learned about what my abdominal gap was and how to fix it. Now if I could just find the time to do those exercises... ;) Congrats on baby #2!!!

Pick up a Core Pilates video. That includes many of the ideas already listed. Start with 3 times a week and as you get stronger, add in another day. At least that's what worked for me - I'm built slender like you, and needed help getting that baby area under control. Is there a Womens Center in your local hospital? In my area, I am able to take a 1x per week Power Abs class in the Womens Wing of the hospital. It helps to be around other ladies working on the same area. Good luck!