Nipple growth is a good indicator that your dog is going through pregnancy changes. The nipples of an unbred female are usually quite small, and the area immediately beneath them is flat. However, once a pregnancy is in progress, breast material will develop beneath the nipples, which will also increase sightly in preparation for eventual milk production.
Milk production will begin in earnest and colostrum, the first milk, can be gently expressed from the nipples, sometimes leaking a bit on its own from the 7th week of pregnancy and onwards.
If your dog is leaking breast milk, then you should take her for one final vet checkup, and start preparing for the puppies by setting up a whelping box.