Java went into heat about 2-3 weeks prior to her spay appointment. then back out - but back in again... just in time for her spay
Firefox went in, seemed [to me, anyway] to go back out in a week - but her spay was right after that, & they said she was still/back in heat. she wasn't showing any signs that time, tho.
my others were all spayed prior to going into heat.