What is the due date accuracy w/ 1st trimester ultrasound?

It depends. It depends on what was measured and when. Very early, the sac diameter is accurate to ±5 to 7 days. Later, a measurement called the 'Crown-Rump-Length' at 7 to 10 weeks of gestation is the accurate to ±3 days. After 10 weeks, the accuracy of the CRL falls slightly with a margin of error of ±5 days at 10 to 14 weeks.