I gained 35kg during my pregnancy thanks to an addiction to banana bread and babybel cheese. When I asked my Obstetrician what was "normal" for a twin pregnancy, she said I was "on the high side." Turns out I was about 20kg+ on the high side. But alas I was eating for three and it was the only way to keep the morning sickness at bay, so I endulged.
As I gradually stretched to the size of a small whale (*photo evidence below), I was amazed by how fit mums of toddler twins were. I often asked how they managed to get back in shape after popping out two watermelons and most responded with "... just wait until your twins start walking!"
Sure enough chasing after two kids that seem compelled to run in opposite directions negates any need to do formal exercise. Besides, your stomach can only stretch so far before it will look like a shar-pei's face so I personally don't see much point busting my ass in the gym these days (not that I have the time anyway). But below are a couple easy exercises I do at home mostly for the kids' enjoyment... and to avoid incontinence.
1. Planking. So your pelvic floor muscles were not designed with 6kg of babies in mind. Planking will not only help you not pee yourself, it also provide a perfect tunnel for the kids. Just hold it long enough not to the squash them.
2. Press ups. Build upper body strength essential for carrying two kids at once when they don't feel like walking anymore. Remember to keep your mouth closed to avoid tasting the tuna sandwich they had for lunch.
3. Leg lowers. A favourite in our house. Great for core strength especially when both kids want a turn simultaneously.
4. Stretching. End your workout with a good stretch like downward dog (and puppy).
Remember, exercise will only get you so far. Best to combine with a healthy diet unlike the fish fingers on toast I managed last night.
* Me with my hobbit feet the day before giving birth at 35 weeks. As I said, you can only stretch so far.