How accurate are transvaginal ultrasounds in dating conception date if have irregular periods?

Most Accurate. Transvaginal ultrasound is the most reliable way to determine a pregnancy due date. When done early, it can date a pregnancy within a few days and is more accurate than determining a due date of the last menstrual period.
Very accurate. The dating accuracy is within +/-3-5 days before 6 weeks, +/-5-7 days before 13 weeks. Take Folic Acid 1-5 mg daily!
Very good. Ultrasound uses measurements not dates to estimate the date of conception.
Pretty good. Transvaginal ultrasounds, especially early in pregnancy and repeated can be pretty accurate in determining the length getation and thus can calculate the date of conception.