I know they count the weeks of pregnancy by the date of the last period. So if the last period was on Dec. 1st for example, Jan 1st would be sometime around week 4.
But what about those with longer cycles? With cycles that are 50 days long for example. So if two women have their period on Dec. 1st, one with a regular cycle and the other with a longer cycle, I believe that they didn't conceive on the same day.
Am I right? Please explain.
It is counted exactly the same, however it's more likely that the estimated due date might be off by a week or so if you have an irregular cycle. Your doctor will have a better idea after your ultrasounds.