If she's pregnant and it's her first child, she's not yet a "mummy". I don't know what a "water birth level" is. However, I will use the term as it appears in your original until/unless someone corrects it.
The pregnant lady has been told that the water birth level is too low.
The pregnant lady has been diagnosed with low water birth level.
"Less" is definitely incorrect. That's a comparative. You need an adjective.
Student or Learner