You used to punt weekly, now it's down to once every three months. Meanwhile, you used to fuck the wife several times a week, now that she's pregnant it's once a month.
Not rocket science to figure out what the problem is. Your wife is still keen, you're presumably the one responsible for her condition, why can't you face sex with her anymore? And if full penetrative sex isn't arousing, there are other options. Watch some porn with pregnant females and get some inspiration.