Okay I'm 18 and pregnant ill be having the baby at 19 but I don't know when I got pregnant and how far I am, my last period started Dec.14 of 2011 and I missed my period which was suppose to be on Jan 10 or 17 but my boyfriend and I didn't have unsafe sex in Dec. And the time we did was during after my period and a little bit before the next and also during that time we had done it twice before I had missed my period on Jan 10 or 17.. sounds confusing but I just want to know cause its fustrating me.. and I haven't been to the doctor but soon I will.. I just before I go I want to find out or at least an estimation cause I've been doing these little calculators on the internet but I just don't get ovulation, fertile and conception and phase of I don't know etc.and I been doing research and I couldn't find any one with the same situation as me..

There is no way to know and find an "exact" date. If there is not a issue of more than one possible father, it does not really matter.
Some women can get pregnant while on their period, and of course women can get pregnant using condoms or birth control, None of them are 100 percent effective, and even 99.99 percent effective means you can be that one in 10,000 that will get pregnant.

The doctor will give you a estimated due date, so normally conception is give or take two weeks of that date.