Phase?

I'm 17 weeks pregnant with #2 and for the past few days, the only way my 16 month old son will fall asleep at night is by straddling me. I can move him and he's fine but he tries to do it daily during the night too. It's cute and all, but he's a little heater and it puts strain on my back (thank god i'm not that big yet).. Is this because of the pregnancy? Or just a mommy phase?