i quite agree with you. freaks a lot of fathers out. but, from the first day of your last period it is. maybe because conception has quite a timeframe where it can occur after sex...i don't know. i'd prefer if they used a likely conception time as the start of pregnancy though.
its virtually impossible to know when exactly conception would have occurred. you can have sex one day and not have conception take place till another day. sperm can stay alive for awhile.
its easier to determine when your due by your lmp. sometimes women dont know when their lmp was but they can tell about when by an ultrasound or by a blood test
When your egg is fertilized it can take 6-12 days for it to implant, so it's really hard to tell the exact moment. Only after implantation does the egg start releasing the HCg hormone. So they base it on your LMP, and 40 weeks later is when you are due. They can change your due date after you have an ultrasound in the early weeks to tell how far along you are by measuring the baby. So it is confusing, but in the end it turns out right.
I used the online calculator to find out when I was the most fertile and then did some more research to try and slam dunk a pregnancy. It gave me 5 days of fertility. Days 13 - 17. That's with a 28 day cycle. But my 28 day cycle was forced by birth control, so I'm not too sure if it's a wrong estimation. And like smjmekg said, it can take 6 - 12 days to fertilize the egg. I also found out that our egg only lives 24 hrs, and that a man's sperm can last 48 - 72 hrs. One thing I was wondering was, if my period was wacked before I started birth control, should I have used those time spands when I did the online calculator? Cuz before I was on birth control, my periods would go anywhere from 28 - 32, or sometimes even up to 40 days apart, from the last day of my period. Please help!!