Good luck with those , I pcked up a 'pregnant' feral last night - Haven't got close enough to check for sure yet - It'll probably turn out to a be tom! 
I did that once, Tiger was the first feral I ever caught, didn't know a lot about cats so listened to the advice off a friend of mine who got me involved in the first place. We spotted a cat with a "bulge" at the rear.......thought possibly a pregnant female
Kept "her" for a week as "she" had the snuffles and then got her in the vets for a check up........vet laughed when I said can you check that she's pregnant, she said he'd have a job cos he's got a pair of
Since then I've always erred on the side of caution when I've taken them in
On the plus side mum and babies are all doing fine......she's started meowing at me now and has let me check they're ok from time to time
