How old is a fetus when you find out pregnant?

It is not possible to know the exact age of a fetus when you first discover you are pregnant.

The pregnancy age is usually estimated based on the first day of the person's last menstrual period (LMP). This is known as the gestational age. The gestational age is calculated by adding 2 weeks to the number of weeks since the first day of the last menstrual period.

For example, if the first day of the person's last menstrual period was January 1, then 12 weeks after January 1 would be April 23. Adding 2 weeks to April 23 gives a gestational age of 14 weeks. This means the fetus is estimated to be 14 weeks old based on the gestational age.

However, it is important to note that this is just an estimate and the actual age of the fetus may be different. The estimated gestational age can vary depending on the accuracy of the person's last menstrual period and other factors.