Thank you for your query.
test would be positive on the first day of the missed period itself.
THat is, around 4 weeks, as the pregnancy age is calculated from the date of the last menstrual period, and not the date of conception.
Hence, by 20th February, you could have been 5 weeks pregnant as per transvaginal ultrasound
, because there is a few days error possible in measurement.
To know the exact gestational age
, you need to specify the date of your last menstrual period, and then corelate it with your next transvaginal ultrasound when the fetus also would be seen ( after 6 weeks pregnancy )
take care, and do not freak out as long as everything is normal