I have an irregular menestration, 2-4months between periods. how do i calculate my conception date?

Ultrasound: When you have an irregular menstrual cycle, it is very difficult to pinpoint your due date. Even if you are irregular, we would date your pregnancy based on the first day of your last menstrual period (lmp) unless a sonogram in the first trimester disagrees. In my practice, i would do a sonogram to establish the dates around 9 weeks or so based on your stated lmp.