Doctors ALWAYS calculate a pregnancy by the first day of your last period - no matter when in the month you got pregnant. Usually that gives you 2-ish weeks free. So when you test for pregnancy, you may only have been pregnant for 2 weeks, but are considered 4 weeks pregnant.
There aren't any statistics for what you are asking. There isn't enough data on when women ovulate. As a general rule, women ovulate approximately 2 weeks (14 days) before their next period is supposed to start. If you have a classic 28 day cycle, that works out to be right in the middle.
If you got pregnant 7 days before your period, start testing every few days your period is late. Pregnancy tests can detect hormones as soon as 10 days after conception.
