Those with ultrasound

I ultra-sounded my girls and it got me thinking…

Now that I have a baseline, how often are people rechecking (once every couple weeks, once a month, etc) and at what point do you stop ultra-sounding?

I plan on pairing when they hit around 18-20mm … theoretically that would be at least 2 pairings prior to ovulation I think. Do you continue doing ultrasounds after you pair?