Holly - It is a little early to use notching as an alarm bell as it is sometimes just artifact at this early stage.
A single artery happens sometimes with no other problems - sometimes it is due to other issues with the baby (sometimes cardiac). All of this though is an unknown quantity right now.

Right now your litle one is growing and in another month or so a more detailed u/s will give a more accurate picture.

Wishing you lots of support along your journey - remember we are here to support you.