When did you conceive if your due date is February 13?

May 21 of the prior year would be the approximate date, plus or minus a couple of days or so, as the issue is rarely so well determinable from such an uncertain time as a projected due date, because it is not clear how your due date was determined, and how accurate it is.

See http://www.vaughns-1-pagers.com/biology/gestation-periods.htm.

Nine months less 8 days from conception to delivery is typical (266 days). Thus counting backward nine months from February 13 gives May 13 of the prior year, less 8 days makes the date of conception about May 21 of the prior year, but it could be a couple of days either side of that in actuality.

One can also calculate 280 days, or 40 weeks, from the first day of the woman's last menstrual period. See http://www.betterhealth.vic.gov.au/bhcv2/bhcarticles.nsf/pages/Baby_due_date?OpenDocument

but this does not give you the date of conception, it only relates the due date to the first day of your last menstual period. As this website noted, only 4% of births actually occur on the date predicted.