How Pregnancy is calculated?

sounds weird but i want to know it.

how does one calculate pregnancy weeks?

a. From day 1 of last period n it is week 1.

b. From 1Dpo?

location - India

Last time when i was pregnant, i got to know it at 6 weeks via home test. i had long cycle back then n didnt have period for 2.5 months. When doctor confirmed via scan i was 6 weeks old, she also told me baby had the healthy heart beat. If that was true, i took a home test around 2.5 weeks n it got me Negative.

i had shortness of breath, extreme feet cramps back then n i had 0 idea of pregnancy then. i realised they were symptoms after knowing i was pregnant. FYI, my mom had same symptoms wen she was preggo with me.

Does baby heart beat start around 6 weeks n are those 6 weeks counted from 1dpo?

this time i am very regular with periods n doesnt seem to get vvvvvfl at 10dpo 🤣

help me understand !!