The problem with going by the date of your last period is that it assumes you ovuate exactly 14 days later. Some people don’t ovulate right on schedule. I ovulate about 3 weeks after my period. Some people ovulate sooner. If you know the exact day you ovulated bc you were using ovulation tests or temping then it is more accurate than the first day of your last period. However if you are just going by when you had sex but don’t know exactly when you ovulated, you should go by your period bc you could have ovulated up to 5 days after having sex and still get pregnant

Your OB will usually go with first day of your period unless an ultrasound says otherwise. My OB has changed mine in the past because it was more than a week off. So for now, just go with period dates until your OB chimes in.

It goes by the first day of your period. That starts the cycle.

If you for sure know date of conception (ovulation) then that will be more accurate