The time of pregnancy is calculated from the first day of your last menstrual period (LMP), which is typically two weeks before conception actually occurs. This means the first two weeks of the 40-week count are before fertilization, during which your body prepares for ovulation and a new cycle begins
There is no “new calculation”. This is how medical professionals have calculated weeks of pregnancy for decades if not centuries. The reason is that it’s a lot more reliable than going by date of conception (which is not necessarily the date the parents had sex. Sperm can take days to reach the egg).
Source: I’ve spent 12 years writing accounting and quality management software for midwives.