Congratulations! I was 44 when I got pregnant. I'd had two previous miscarriages but Dr's said I could carry on working out. I really took almost all impact out from the off. To me, it just didn't feel right. I carried on stepping for as long as I could. Eventually, my back told me to stop around 7 months, if I remember correctly. I also did kickboxing (not for everyone when they're pregnant) until the week before I gave birth. I mainly did TJ and KCM's 30 min Kickbox. I found the latter to be the best and it didn't aggravate my back at all. Kelly also has a great modifier in Lauren. I also did Cathe's B & G's most weeks and Classical Stretch. I tried doing Leslie but it seemed to aggravate my back. However, I did try to walk outside wherever possible.
I also worked on my core and would recommend the Tupler technique. My stomach sprung back pretty much straight away (even after an emergency C-section), helped by breastfeeding, and the midwives commented on my strong core! I didn't put on any weight, apart from my DS, Benjamin. I don't know if this relates to how active I was, but Benjamin has always been an active and alert baby, now toddler, from day 1.
Hope that's helpful.
Emma