Hmmm, I don't believe thats true about needing to cut out all exercise in 1st trimester. In fact, I have never heard that before at all!
Its fine to continue with what you were doing prior to pregnancy, so long as your body is used to it, and so long as you keep an eye on your heart rate, keep yourself hydrated, and don't push yourself or overheat yourself.

I gained over 5kg in 1st trimester this time around. Due to cutting out exercise (from being too tired and sick), and also because greasy fatty food was the only food that would make me feel better!
My first pregnancy I gained about 12kg in first trimester, but that was due to me starting off very underweight, and other circumstances.
My first pg I gained 42kg for the entire 9 months.
This time I have gained about 15kg so far, and I am currently 27weeks.