I ovulated on octomber 4th(that was cd13). according to this due date is 06/27.first trimester ultrasound(week 8) said dd is 06/23.which is reliable?

Depends: The answer depends on how accurately you know when you ovulated. The exact day of ovulation is hard to pin down unless you hare having daily ultrasounds by a reproductive endocrinologist. Ovulation prediction tests are not accurate for dating. Ultrasound at 8 weeks is accurate to plus or minus 5 days and is more accurate unless you know the exact day of ovulation by ultrasound. Best wishes