Forgive the cynic in me ...
You have sex with your wife once a year, and in that singular event last year, she conceived, at age 37-38? Although I certainly cannot say it's impossible, it seems incredibly unlikely. Again, I know nothing about you or your spouse, but the first thing that went through my mind is either you are playing us - or you're being played.
She sounds asexual:
Asexual Visibility and Education Network - Home
Although many people may admire the fact that someone remains a virgin until being nearly 30-years-old, I generally assume there are issues attached to the reason. Given the description of your sexual relationship, it does not appear that she was simply 'saving herself'.