Hello! With late ovulation, do they count by the ovulation day rather than…

Hello! With late ovulation, do they count by the ovulation day rather than by the obstetric (gestational) age?? I only had intercourse with my husband on the day of ovulation, and when I was 4 weeks by ovulation day they told me there was only a gestational sac and no embryo yet. They scared me saying it could be a missed miscarriage and told me to come back in 2 weeks... but I don't have any pain anywhere. Has anyone had a similar situation?