10 dpo big BBT spike?

Rebecca✨ • 29,FTM to my 2 year old daughter && coffee-obsessed!
I'm really trying to wait until I miss my period to test but this morning I got a big spike in my BBT. I'm hoping it stays up for a while!!! Is this a good sign tho? I think my temp needs to be elevated for 18? days? Is that right?