Your dentist books appointments - just like the hygienist. If he doesn't know you have a cavity he may not have an opening - he will be spending his time seeing other patients. He does way more than just the last 5 minutes of your cleaning appointment. Where would he "find" 30 minutes to do your appointment if he's with other patients during that time?
You could ask the office manager if you book far enough out if there are any days that he has an appointment open right after your cleaning appointment. They will sometimes do that.
If it is a smaller office, or a slower office - then the dentist will have time to fill the same day.
He wants xrays to make sure of the depth of the decay. If he starts a filling and the cavity extends down into the root and it's too big to fill, then you have to have an impromptu root canal. With xrays he will be able to tell EXACTLY how big, what the best course of treatment is and how much time he will need for the appointment.
Cavities actually have NOTHING to do with oral hygiene. It has to do with the acidic level of your saliva.
While you are pregnant end each meal with string cheese (lowers the ph of your saliva). Also - chew gum with xylitol (like Trident). There's a reason 4 of 5 dentists recommend it.... the xylitol also lowers the ph. But it has to be sugar free gum with xylitol as the first or second ingredient.