I have a female dog and is about 10 months. Well I took her to my friend's house and she has a male dog who is about a year. And they were outside and I hear a dog scream so I go and check. It seemed like their butts were stuck together and his penis is inside her. I believe she was/is in heat. That happened two times and the first time I found them like that and the second I stopped them and they stayed like that. What are the chances she's pregnant.

Very likely.

Very likely.
Even though it's their first time for both dogs? They are both Chihuahua mixed

Very good chance.

Take her to the vet, they'll check her and you can get a spay done if she's not, and a spay abort if she is. She shouldn't have been allowed near a male dog when they're both still intact.

At 10 months she's way too young to have puppies. If the puppies survive, she likely won't, or she won't be able to, or willing to, care for them.

It's really best to take her to the vet and not only abort any puppies she may already be pregnant with, but prevent the chance of her getting pregnant in the future.

Just like people, first time too. Get her spayed if you don't want puppies all the time.

Even people can get pregnant first time. I don't know many, if any dogs, that didn't get pregnant unless one was fixed.

Even though it's their first time for both dogs? They are both Chihuahua mixed

Doesn't matter if it was the first time. Why would you think that would make a difference?

Chihuahua... that makes a spay abort even more reasonable. Chihuahua's, because they're small dogs, are known to have issues with pregnancy and giving birth. If she is pregnant, and you decide to let her have this litter, she'll likely need a c-section. That will cost you a few thousand dollars. Bottle feeding will cost even more, not to mention the time it takes.

I'd get her to the vet and do a spay abort.

If you saw it happen the first time, why would you allow it to happen again? AND if she was in heat, why'd you let them out together... this is some major owneritis... ugh