Beginning of pregnancy and conception by: Anonymous
It is difficult to know the exact day of the beginning of pregnancy and conception. Most of the time the date of conception is an estimated guess. One way that people measure the beginning is to start from the first day of their previous period (the last regular period before they missed their period). So the first day that your period started last month, would be the beginning day of this pregnancy. This is called the "gestational age" of the baby. An ultrasound exam at the doctor's office can be helpful in determining what was the beginning of your pregnancy.
If a woman knows for sure of a specific date she was intimate then she may know the day of conception and that would tell the "fetal age" of the baby.