hi kirsty,
i'm sure that everything will be fine.
i didnt find out that i was expecting until 20 weeks when i went for a usual dating ultrasound.
all the while i had been drinking and taking medication, and everything looked fine.
my sister in law couldnt quit her smoking while pregnant, but did cut down and she had a perfect little girl.

a little anxiety is normal, but try not to let it ruin what could be the happiest day of your pregnancy. you might even be able to see what you're having!
take care