Clue: In obstetrics, is it best to ignore the odds and take no action? (4)
Answer: STET
Explanation: Definition: take no action?
In 4D, we had one wordplay and two definitions; here we have one definition and two wordplays: a hidden answer in 'ob**STET**rics'; and 'i**S** i**T** b**E**s**T**' minus letters 1,3,5,7 ('ignoring the odds').