3+ weeks means you conceived over 3 weeks ago, so by “official dates” you would be 5+ weeks pregnant. My periods are also totally irregular, so I feel your pain with the difficulties of that. The only way to know for sure without having a solid date for last period, or for ovulation, is a scan. The good news is if you give them the date of your last period and your cycles are long, they’ll automatically assume you are further on (as they use the 28 day cycle model for everyone

) so you should get a scan sooner rather than later. What was the date of your last period?