I had to do this, too. I'm only 30, but my mom's had breast cancer twice (and the first time was at age 37).
I actually ended up weaning anyway (kind of mutual with DS but upped the timetable a little due to the mammo need as recommnded by my doc - DS was 28 months). When I got the routine baseline mammogram, I didn't ask the tech about it. I ended up having to go back for a follow-up (which was all perfectly normal) and that ended up being several more mammograms and then an u/s. I asked the doctor who did the u/s about it (mostly because I think she was the only actual MD I saw through the whole experience) and she said they CAN do it while you're nursing, but they don't get nearly as clear of a picture, so you really have to weigh your odds: your age, your family history, and your relationship with your child.
I can say that I personally feel better having a "clean" baseline and now that I'm pregnant again, I know when I go back yearly I'll still be nursing for the next 2 years or so...I can sleep better at night knowing I didn't miss anything, you know? I don't mind having a slightly obscured scan as long as I know I'm not just a time bomb waiting to go off, if that makes sense. If DS had really struggled with the weaning, I would have reevaluated, but it was pretty painless for both of us. Do I feel a little ripped off about having to wean earlier than I would have normally? Yes, but I guess that just goes along with the cancer territory - I kind of feel ripped off that it's disturbed my mom's life so much and that I have to worry about it forever. This is sort of how I came to feel as at peace with it as I could.
