The first puppy in either the right or left horn of the uterus is lifted into the birth canal and then detaches from the wall of the uterus. When this happens, in a normal delivery, the puppy will seem to "pop" out sometimes all at once, sometimes halfway and it takes more contractures to push the puppy on out. If you see a puppy that looks hung up, grasp as much of the sac and puppy as you can and gently pull outward and down when the female has a contraction. Never pull the puppy straight back or jerk it hard.
Puppies can be born head first, feet first or butt first. The easist delivery will be head first, and is the most desirable delivery of course. Usually these come out without problem. If the puppy tries to emerge feet first or butt first, it sometimes gets stuck and takes longer for mom to push it out.
At this point, do not panic, but just try to gently pull outward and down using a dry washcloth to get a good grip. The sac on a puppy is very slippery. As soon as the puppy is out, the mother should by instinct start to lick and tear at the sac around the face, eyes and nose. This is of most importance at this time. If the sac is not broken quickly, the puppy could smother to death. So if mom does not start this right away, just simply tear it with your fingers, and the puppy will begin to try and breathe. If mom does her job correctly she will continue to tumble the newborn around, licking and cleaning the sac completely off to stimulate the puppy to breathe. Again, if mom is too tired or for some reason her instincts do not kick in quick enough, you can begin to stimulate the puppy to breathe on its own by rubbing it briskly with your dry wash cloth, rub its back and shoulders, head, evey part you can get hold of and hopefully mom will also begin to cut the cord about this time and consume the placenta that should have arrived along with the whelp or shortly after. Mom does not always do this however, and I have seen mothers let their whelps lie with their placentas and cords attached until they deliver two or three other puppies, and then turn to eating the placenta and cutting the cords. As long as the puppy is out, the sac is broken and they are not struggling to breathe, and are breathing well on their own, it does not hurt anything for them to lie attached to the placenta until mom gets good and ready to return to that part of things.
I think sometimes mothers get distracted by the others that are trying to be born and they know if their previous whelps are out of the sac and breathing, sometimes even nursing with the placenta and cord attached, they are fine. If mother is doing her job correctly she will always stimulate them to breathe on their own as first course of action and leave the other things to another time if necessary.
She needs to deliver one placenta per puppy and you need to keep count. If she retains a placenta, it can cause infection, and you will need to take her to the vet for a shot to help her to expel the placenta. It is always a good idea to take your mom to the vet shortly after whelping to be sure she has not retained a placenta or even a dead puppy that will need to be expelled.
Anytime you leave newborn puppies or take their mother away for bathing or going to the vet, you will need to provide the puppy box with a heating pad and place a baby receiving blanket over the box to create a warm incubator. I learned this lesson the hard way. I took a mother from her babies just long enough to bath her and blow dry her, but never provided the newsborns with a heating pad, it was summertime even, and all those babies died within a day or two, they were doing fine before. I concluded they must have "chilled" while mom was gone. Mom will provide a lot of warmth for her newborns. She will wrap her body around them to warm them up, but you still need to use heating pads, and especially if you take mom from the box to tend to her for something. The number one cause of a newborn dying is becoming chilled. It is imperative to warm them up as quickly as possible and keep them warm.