If the radiologist is competant enough, ultrasound can be 99% accurate.
Its pretty acurate. If not completely it will be off by a day or so but not as much as a month.
Maybe around 2months and 2 weeks.
At 7 weeks an ultrasound is accurate within 2 or 3 days. At 20 weeks it is accurate within 5-7 days.
Generally under 12 weeks because after that it gets more complicated
Depending on where you live you can get up to the 7th month. But don't wait that long. I'm adding a link to where you can see what applies to your state.