How do I know how long I've been pregnant?
You are pregnant, but for how long? If you are pregnant, you also want to know how long you have been pregnant. You will not get an ultrasound immediately, so how can you calculate how long you have been pregnant? To know for how long you have been pregnant, you must of course first be sure that you are pregnant. If this has been determined with certainty with a pregnancy test, you can calculate how long you will be pregnant. How do you do that?
Why don't they make an ultrasound more often in the hospital?
Because an ultrasound is expensive, and because it also takes time for medical personnel. After all, the majority of pregnancies proceed normally. Most children are born healthy, which is why multiple ultrasounds are only necessary if major or minor problems in mother or child are taken into account.