I'm 7 weeks and 4 days pregnant when did I concive ?

Wen was your last period ?

In determining a due date we need to know your last monthly period. Conception is more tricky than that. If you are questioning paternity you will have to wait until the baby is born to have a DNA test done.

My last period was 12-15-2011

If your LMP was 12/15, you most likely conceived on or around 12/29. However, remember that dating conception is not an exact science and has marginal room for error.