If you are 5 weeks pregnant what happened 5 weeks ago? mplantation? fertilisation? intercourse? or your last period?

Last menstrual perio: The number of weeks pregnant is officially counted from the beginning of the last menstrual period prior to getting pregnant. The dating can be based on a 28-day menstrual cycle, especially if a woman has irregular periods or doesn't remember her last menstrual period. If pregnancy is confirmed by a urine or blood test, an ultrasound scan done at 8-11 weeks can confirm an original dating estimate.