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It's really, really easy to miss a placenta or two during whelping. Not seeing them doesn't necessarily mean she has a retained one, and likewise having them all pass doesn't necessarily mean there's no risk of infection.
Placentas can be eaten in a flash (and I'm talking one lick), expelled during potty breaks, or disintegrated into unidentifiable pieces that are expelled instead of a whole placenta and just look like regular or chunky discharge (either during whelping or in the days after).
Monitoring your dam is key
Regardless of whether or not you see all the placentas, you want to do the same thing: monitor her. Take her temp at least twice a day, watch her appetite, how she feels and is caring for her puppies, other behaviors, signs of discomfort, odd, excessive or foul-smelling discharge, etc.
Continued straining and restlessness after labor is possibly the best sign of a retained placenta (this assumes adequate calcium is given).
(We no longer give a "clean-out" shot. We stopped about 5 years ago. No infections.)
What causes infections (pyometra and metritis)
Infections aren't caused by the placentas. Infections are caused by bacteria entering the uterus. Which is wide open to invading microbes during whelping (your best prevention is cleanliness during whelping and in the days after). So they can happen regardless of retained placentas or not. A retained placenta is good "food" for bacteria, but not the cause. So if there's an infection and a retained placenta is suspected, then vets are likely to give oxytocin to help expel it. They will still give antibiotics at that point.
Occasionally a retained placenta can be a problem, but not as often as we may think. Many repro vets have changed to waiting for a problem before giving oxytocin. You'll want to talk to your repro vet about this to decide the right course of action for each situation.
The role of oxytocin
It's also important to understand what oxytocin does. Oxytocin increases the FREQUENCY of contractions. So that's why it's useful for treating a retained placenta after whelping. But calcium is what's needed for the STRENGTH of contractions.
So giving calcium during whelping gives her good, strong contractions to help expel both the puppies and placentas. She also has contractions in the days after whelping to help expel anything that might be left and also to help the uterus go back down to normal size. Again, calcium is very helpful for this.
See my posts about calcium and oxytocin for more information. Intro to the roles of calcium and oxytocin during whelping and how to use calcium during whelping and nursing.
[Oxytocin can help bring on milk if she's not lactating, but there's a short window for that after whelping. And it's given in very small doses.]
Oxytocin is given these days in much smaller doses than we used to give it back in the day. Like 0.5cc instead of the up to 5cc, which is more likely to cause uterine rupture.
If you want the effects of oxytocin with much reduced risk, nursing is a great way to induce her body to release more oxytocin at a healthy and more sustained natural "dosing." If puppies won't nurse or there are other reasons nursing isn't happening, you can stimulate her nipples for a similar effect. [This is why allowing nursing between puppies during whelping is important and helpful.]
How a reproductive vet can help
As usual, my best advice to you is to find a good, collaborative reproductive vet and consult her on matters such as this. If possible, ask about these things before they happen. Ask about her view on oxytocin and when and in what does it should be given. Also ask about retained placentas and how she prefers you handle those.