Can dogs get pregnant without a tie?

So my friend and I have been trying to breed my Pug female to my friends chihuahua male. There hasnt been a tie but it has gone inside a few times. Someone told me they could still get pregnant without a tie but others said they cant. If they cant well keep trying.

The reason we would really like pups from them is because on a walk we saw the most adorable Chug. She looked exactly like a pug but tiny like a chihuahua. I fell in love with it and since my friend owns a male chihuahua and my Pug is 2 and a half we though wed have a litter.

We are planning on keeping one each and the rest will go to family members. I know small dogs dont have big litters. Im hoping one of our chugs looks like the one I saw on the walk.

So can the bitch get pregnant without a tie?