Pregnancy scares are the last thing any 18 year old girl wants on their plate. This is not the time to get into any panic attack. The first thing you need to do is get a home kit to test. They are usually very affordable and can be gotten at the nearest pharmacy store.
If you get a negative result from the test kit you have the option of having to watch your body some more for a period of time, or still going to the hospital to have a proper test performed and have peace of mind.
If however, after testing at home, you get the positive sign, you need to alert the father or man responsible for the pregnancy. Next, you have to go to the doctor to do a confirmatory test because sadly, these tests are not fool-proof.
After doing a confirmatory test and it turns out positive still, the both of you would need to discuss what your options are. Now, this may or may not be in the presence of your gynaecologist. But it is always good to get the input of the gynaecologist as he/she would advice you with reference to your age and how far gone with the pregnancy you are.
Also, you would need to talk to your guardian/parent (or older mature adult if the aforementioned are absent). This is because, at this time and depending on what you decide to do, you need all the help, support and love you can get.
This is especially if you are in a society where there is still a lot of stigma associated with being a single mother. You may choose to have the baby and raise it, or have it and give it up for adoption. If you are in a society where abortions are legal, that can also be an option for you. Bear in mind that “18 and pregnant” is not the end of the world.