How could a doctor find out how advanced her pregnancy was?

Doctors and midwives take all sorts of measurements during pregnancy to determine the baby's progression. The sonogram is the most advanced method. They measure the bones and see how developed the organs are.

It is important to know to determine when approximately she should deliver. If complications were to arise and she were to start labor, the doctor would need to know how many weeks the baby had developed. Less than 25 weeks, and it is not likely that the baby will survive outside the womb, so corrective measures would need to be taken. After 36 weeks the baby is usually safe and will often be born without problems. There would be no need to try to stop labor at this point.

Advanced... meaning how far along she is?

Well, it's important to have an idea of when the baby is due of course. Also, alot of prenatal tests have to done around a certain time. The test for birth defects can only be done between 15-19 weeks, and the glucose test to check for gestational diabetes should be done between 24-28 weeks.

Doctors need to know to be able to measure the babies growth, to make sure his/her limbs and everything are growing at a normal rate, and also to be prepared in case anything goes wrong with the pregnancy. If a woman goes into labor and doesn't know if she is 6 months or 9 months that makes a BIG difference in what steps doctors would take.

It is very important to know how far along the pregnancy is for the health of the unborn child as there are certain check ups and tests to be done at different stages of the pregnancy also to know roughly when the baby will be born so its not rushed leading to more stress for the mother and baby which could lead to complications through the birth

Aside of helping mom and baby and dad prepare for all the changes that are going to be happening, an expecting mommy has a barrage of tests to go through in preparation for that baby. Some tests can only be done between 14 and 18 weeks gestation, some tests have to be done at 20, a few at 26, ect. Because of the sensitivity of many of these tests, it's very important to know as close to accurately as possible how far along you are.

Knowing the age of the fetus is very important. You wouldn't want a surprise due-date because of not knowing how far along mom is. (I know some people get surprises either way.)

Tests are to be run at certain times during pregnancy as well. Certain blood work, certain dates for measurements, etc.

A sonogram is one of the ways a doctor can find fetal age, taking measurements and going by an average of size. Very simple proceedure, nothing to be worried about.

Lots of reasons! They want to see if the baby is developing normally, and check the progress.

Also, there are some tests that can only be done at certain times during the pregnancy.

When they do the initial ultrasound, they can pinpoint almost to the day how old the baby is, and when it is due.

I'm sure this is helpful for insurance reasons, too.

They could tell a couple of ways. External exam tells the doctor about how far along she is. A blood test will tell down to the week, based on hormone levels. Usually, though they just add 14 days to the first day of the last period and call that the conception date. There's a chart and everything.

In order to estimate when the baby will be born the doctor needs to find out how far along the pregnancy is!

The doctor found out how far along I was by doing a LMP (Last Missed Period) ultra sound. I have PCOS (Polycystic Ovarian Syndrome) and have gone years without a period. Doing this LMP helps them know the age of the fetus, and what tests should be done, and what should be done to care for the unborn child. Also its a good idea to know the due date.

For testing
Know that feotus was developing properly
Determining a due date, if you go over - how long to let you go before you could be induced.
Some women need injections due to their blood type, this has to be done at a certain time.
And it a nice to know kind of a thing too.