I also have irregular periods. I skip months some times. I had a dating sonogram done at my first appoinment to find out how far along I am. It seems to be pretty accurate.
Mine was 2.51 which they said was normal. I also had harmony test which came back negative.
Just keep counting by the first ultrasound...same happened to me i did that.....
Go by your last period. count how many days between the miss period and previous one. I conceived march 2nd