The only way to know for sure, is to call them directly and ask. Be honest and explain your questions and concerns nicely and completely up front. Ask for their best advice as to how to proceed.
I would think that since you already have coverage and are a paying customer there would be no 'pre-exsisting' condition since you have been with them before you got pregnant...but I am no expert and in my experience Insurance doesn't EVER seem to benefit us, it only goes in their favor, ya know?
Honesty is always the BEST policy, that way you are not trying to be sneaky or under-handed. If you go in stating the truth and are just asking how you can change things to benefit you the most, *hopefully* you will get a nice agent who answers and is willing to work with you, not against you! You are a paying customer, there is nothing wrong with trying to get the very best coverage for the very best price, you just need to be open about all of it!
~Since you have yet to see a doctor, technically your pregnancy is not official yet or confirmed...you could always state that you *think or hope* that you might be pregnant and inform them you haven't even seen a doctor yet, hasn't been enough time for a missed period ? Or something?? and then go from there on when and how you should proceed IF it does turn out you are actually pregnant? "I don't want to count my chickens before they hatch so-to-speak but if it is true how should I proceed to get the best coverage from you"?? Something like that maybe?!! But in your own words of course and still maintaing the truth!
Good Luck!