When you see the long-awaited two lines, which indicate a positive pregnancy test, the first thing you want to do is find out the gestational age.
In obstetrics, the gestational age is calculated not in months, but in weeks, and, in addition, the obstetric period is calculated not from conception, but from the first day of the last menstruation. That is, the first day of menstruation is considered the day of conception.
You will rightly note that this method of calculation is not accurate, because ovulation occurs on average 2 weeks after the start of menstruation. But this method is universal and applicable to almost any woman, subject to regular menstruation. The error is not significant and averages 10-14 days.