I am qualified to give stats, but I go to the gym, spin, body balance or swimming once or twice a week.
But I wanted to know about how far to push myself, what was safe etc, so I booked in for a review, with a pregnancy trained person, at the gym and I found it really helpful.
We looked at how to set up the bike to account for the inflating belly, how to monitor my heart rate and keep my "enthusiasm" under control!
Pre-preganancy, I was going to the gym three times a week most weeks and doing two classes back-back or one class and the gym or swim. So I have had to really reign back and remember I am there to maintain my fitness, but not to get fitter, or lose weight. Now I go two or three times a week, but only manage one class at a time. For spin classes I try not to let myself sweat too much, if I am getting too hot, I back off a little. I also force myself to drink a whole bottle of water through the class and another afterwards, obviously not in once go :)
I am 21+4 and find if I don't go to the gym, I really struggle to sleep, which is challenging (with insomnia) as it is. But more importantly, it makes me feel human and really helps to make me forget about the stress of the office.
My local hospital also runs a physio session for mums to be once a month. It is a one off class to talk about PGD/SPD (which I very occassionally feel!) and pelvic floor excercises. It covers little tips like how to get in and out of cars and bed, and how to monitor yourself when doing excercise. I found it helpful, but I know not every Trust offers this, so if you are interested in something like this, ask your midwife. I am in Bristol.
So apologies if this isn't what you are after, and for the long post, but the crux of my advice is book in with your gym and specify you want someone who knows about being pregnant, then they can talk about a fitness programme to suit you.