The most common and most likely answer to this crossword clue is the 9 letter word PRIMIPARA
I've seen this crossword clue in The Times
P
R
I
M
I
P
A
R
A
More clues leading to the result PRIMIPARA
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The straight part
We know that "new mum" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case new mum = primipara.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does PRIMIPARA mean?
PRIMIPARA noun- A woman or female animal during or after her first pregnancy.
- (specifically) A woman or female animal that has carried a first pregnancy to a viable gestational age.