Hey Rach - Congratulations!!!!

The cycle is the number of days from day 1 of AF to the next day 1 of AF.
Luteal phase is the number of days from when you O until day 1 of AF.
Does that make sense.

Generally though the doc will work out the date from 40 weeks after the first day of your last cycle - if you know when that was of course.
Otherwise they will take bloods to give a level to estimate and send you for a dating scan.