UK Visitor visa Query

[Amruta]

UK Tourist Visa query

Hi,
My mother was here in London from Mid March to Mid June ( i.e 3 months ) this year on visitor visa . I am expecting my baby in January and want her to come at the end of December / January. She was issued a visa for 6 months in her previous visiti from Feb-August 2017.
I have couple of doubts on this :

1.Is it ok to apply visa for her again in November ?
2.Is there any specified time span between visitors visa ?
3. Can i describe the reason for her visit as my pregnancy and i want her to be with me during my delivery.

Thanks in Advance