Can an abdominal ultrasound detect a pregnancy at 9 weeks from possible conception and 10 weeks after last sexual contact? atleast a gestational sac?

Yes: Abdominal ultrasound should detect the pregnancy. However vaginal ultrasound would be more helpful especially if the pregnancy is not as far along as expected or in the event it's not growing normally. Serial beta hcgs ( serum pregnancy tests) done every few days are also helpful. A normal pregnancy will show rising values, doubling every few days. See your provider!