I would like to know how to calculate weeks and days when your pregnant and after how many days does the foetus start to form?

See Below: Pregnancy is usually dated from the first day of your last menstrual period. Even though conception actually happens about 2 weeks later, most women don't know exactly when they conceived (It doesn't always happen on the day of intercourse). The foetus begins to form immediately. By 4 weeks after conception it has a heart beat, and can move its hands. Best Wishes!