Today is july 20th and i'm 12 weeks. does that mean i conceived in may? my periods were very irregular to begin with

Counting weeks: Pregnancy weeks are officially counted from 2 weeks before conception. Thus, if an ultrasound scan done early in pregnancy (for example, at 8-11 weeks of pregnancy) says the pregnancy is 12 weeks on July 20, that means the official start date of the counting was 12 weeks before July 20, and the estimated date of conception was 10 weeks before July 20. The start date is 2 weeks before conception.