Visiting US 2nd time

[curious]

My parents might be visiting us in 2-3 months for my delivery.They already have a 10 yr mutliple entry visa and this is their second visit to US,first being in the last year for my sister's delivery.

My questions are
-wht necessary documents do they need to carry with them
-Wht documents should we send them from here.I read abt invitation letter which we have to sent,wht reason should we give in that.is that the only document we have to send.
-Any questions which they have to carefully answer in immigration.(i am on h4 and my sis has green card and worked for sometime until Delivery)

hope someone will clarify,thanks in advance.

[taco]

No need to carry extensive documentation. Likelihood is they won't ask for any papers ; they will just ask them the purpose of the visit and duration they want to stay for. But still, send them a basic letter of invitation
and a copy of your visa, just in case they ask. Avoid mentioning baby care.