Due date with having iui

How Is your due date determined when you have a iui?

My first ultrasound is tomorrow where my doctor said the heart beat should be strong enough and talking about my due date and checking to see if there is only 1 baby or twins because if having 3 full size follicles at the time if my trigger shot.

After looking at things online I'm so confused about how far along I should be and when my due date will be.

I can't go off my lmp, but my iui was on April 26th. So does it get based off your iui date or do they add more weeks to that?

Everything I read either says your due date is off your iui date and the other half of the information and forums say it goes off your iui and they add 2 weeks to that.