Pregnancy on the streets is challenging for a cat. Sure, she might have a smooth delivery and a litter of healthy kittens. But it's also possible that there will be complications. The kittens could die in utero, and if she's unable to expel them it can be fatal for the mama cat. She might not have a safe place to give birth and if she loses track of her kittens or abandons them after delivery, they will starve to death on the streets. It's also possible that she's not pregnant at all — that it's actually a uterine infection (potentially fatal!).
The bottom line is, unless you're a veterinarian, you probably can't make the decision about whether or not that cat is healthy and able to deliver her kittens. She needs to see a doctor!
FixNation will loan you traps and train you on how to use them so you can get the strays in your neighborhood fixed, all at no cost to you. They will also decide what's best for the health of any pregnant mama cats. Getting stray cats spayed and neutered is vital in reducing overcrowding at shelters and moving Los Angeles towards being a No Kill city.