I once did some work for these people:

http://www.temperatureconcepts.com/

And I know they're excellent and sincere, and if I was trying to get pregnant I'd certainly have one of their devices. I think it is deliberately designed to measure sleeping metabolic rate, because that tells you a lot about female cycles.

They might already have the data!

Damn it, it turns out I emailed them about this in November, when I was first thinking about this. No reply. I forgot. I shall get hassling.