Question: It took me a long while to discover you could flip sun visors down (duuh) but I never find anything there

Is there ever anything there?

Answer: 
Boosts regarding having twins take effect at birth.

I have no external references for this. I usually have 10-15 pregnant woman but only one pet for twin boost. I switch that pet around among all my women for lots of babies.

**Updated answer as of May 5, 2018:**

The latest patch made it so that your dweller needs to have the twins pet equipped **before** she is ready to give birth. Same with the stat boost. It is no longer possible to combine +X stat bonus pet with +% twins bonus pet with the same dweller.
According to [The Fallout Shelter FAQ](https://github.com/therabidsquirel/The-Fallout-Shelter-FAQ/wiki/*5.-Breeding#52) by therabidsquirel, after [much experimentation and debate](https://en.reddit.com/r/foshelter/comments/67hvqx/1_special_or_75_twins_pet_for_legendary_dwellers/dgqlbcy/) on the /r/foshelter subreddit: 

> 
> Equipping the pet on a female dweller both for conception (the dance and sex) and for when she becomes ready to give birth (the moment the pregnancy icon appears) will give her a chance at twins or even triplets. Swapping one pet out for conception is easy enough, but if you only have one pet and multiple pregnant women it will be harder to ensure each has the pet equipped the moment they become ready to give birth. The easiest method I can think of doing so would be staggering conception for each woman by a minute or so, recording the order, and then swapping the pet between them in three hours when they'll be ready.
> 
> 
> 

At the moment of birth, you can swap the pet out for another, such as a Child SPECIAL pet.