My last period was march 13 2015 cycle of 31.i took a plan b on march 26. i had an ultasound monday im said too be 8 weeks i. given due date of dec 18 2015. when did i concive?

Conception date: Generally women ovulate about a week after the last day of their period. However, if your periods were not regular prior to conception, then your ovulation timing may be different. Based on your due date and first day of your last menstrual period, you likely ovulated around March 27th, and conceived between March 23rd-31st. Again, this is based on a 28 day cycle.