sept 6
The statement that you are six weeks pregnant means that you have been pregnant for six weeks, hence, you conceived six weeks ago. Bear in mind that these are estimates, they are not necessarily accurate down to the exact day. But they are reasonably good estimates.
5days
When they say that you are 9 weeks pregnant that is a guess. A scientific quess but none the less it is still an approximate guess. It cannot be pinned down to an exact day.
Pick up a calendar ffs.
16th August 2008.
Forty weeks from the first day of your last period. So if you're six weeks, then you are due in 34.
They consider your LMP (Last Monthly Period) to be the starting day of your last period, they track the 40 weeks of pregnancy from that, but no, it's not the actually day where you ovulated and got pregnant. You usually ovulate about 2 weeks AFTER your LMP so technically when you are "4 weeks pregnant" the baby is only 2 weeks.
Its normal
Count back 28 weeks and one day from March 7.
To try and get pregnant you need to add two weeks (fourteen days) to the FIRST day of your period. You also have a two day window to get pregnant.
In two weeks of what? You get pregnancy at fourteenth day of your menses. You can not know the same by this time. You can easily know that you are pregnant by week or ten days after you miss your period.
Twelve weeks and one day before July 7 is April 13.