I am a little confused about how the whole pregnancy is dated and I just wanted some clarification.

I am 19 weeks pregnant now and at my 12 week scan the nurse told me I was 12 weeks and 6 days. That number of days takes me back to the 6th of April 2012. Am I right in saying that at the time I wasnt actually pregnant? That I infact conceived after that date? And am I right in assuming that I couldnt possibily have conceived at any time before that date?

Thank you!