I’m not a breeder, but my friend’s mare developed placentitis so I’ve been doing lots of research for her on the subject of red bag deliveries just in case.
This link has a good diagram that shows all of the parts of the placenta in relation to the foal to help understand what exactly is happening, and also has a discussion about red bag towards the bottom.
http://www.scottcreek.com/Dystocia.htm
This is excerpted from “The Complete Book of Foaling” by Karen Hayes DVM. (I’m transcribing it, so please forgive any typos.)
She calls the red bag delivery a Dry Birth.
"Occasionally there may be a problem with the rupturing of the cervical star. Perhaps the cervical star does not aligh perfectly with the cervix, causing the weak spot in the placenta to lie off center with the opening cervix. Or perhaps the placenta is thickened as a result of a disease process or a nutritional imbalance, and the thickened placenta is too tough to rupture when it protrudes through the opening cervix. Whatever the cause, if the mare should fail to “break the water” before going into serious labor contractions, a dangerous situation will result, and you should be ready to recognize this situation and correct it immediately–early detection and early correction.
In the normal situation, when the outer placental sac ruptures at the cervical star, the inner sack (amnion) should then be allowed to advance and should appear at the vulvar lips of the mare within 5 minutes. Remember that the allantois-chorion (the outer sac) is a dark red, roughened, terrycloth-like membrane, and the amnion is thinner, more delicate and translucent milky-white in color.
If for some reason the allantois-chorion does not rupture as it should, the mare will do one of two things: she will either stop the labor process and start again later, or she will begin the uterine contractions of serious labor and try to push the pregnancy out in its entiety–the foal, the amnion, the amnoitic fluid, the allantoic fluid and the allantois-chorion–she will try to force the entire, intact water balloon and its gallons of fluid out through the rigid walls of the birth canal. This is called a “dry birth.”
A dry birth is dangerous to the foal for two reasons, besides the obvious disadvantage of lack of lubrication. First, as a result of that lack of lubrication in concert with the increase in size of the overall “package” due to the fact that the whole placental unit is trying to come out at the same time as the foal, the foal is being subjected to a tremendous increase in pressure and stress, leaving it more susceptible to physical and neurological damage.
Second, think about the source of the foal’s oxygen–the fetus receives its oxygen not from breathing, but rather from the blood of his dame. When the fetus emerges from the mare, his former oxygen source is cut off, and he must learn to breathe on his own in order to oxygenate hs own blood.
But before he emerges, while he’s still inside the mare, where does the blood in his umbilical artery come from? It comes from the little “roots” of the placenta that are embedded into the uterus, drawing blood from the blood supply of the mare’s uterus. If the allantois-chorion fails to rupture and the mare’s uterus begins to contract, eventually she will manage to push the pregnancy far enough out so that you can see the chorionic surface of the placenta at the lips of teh vulva. This meas that it has begun to detach from the its Velcro-like attachment in the uterus (premature separation), so that at least a portion of the placenta is now no longer drawing blood from the uterus because it is no longer attached. So now the baby is no longer receiving hs full ration of blood and oxygen. As the mare continues to push, more and more of the placenta will coome apart from the uterus, and the baby will receive less and less oxygen. If this is allowed to continue for too long, the foal will suffocate, since it is caught in limbo between umbilical oxygenation and air breathing. And all of this is happening during the time when the foal can least afford to be deprived of oxygen, since he’s in the middle of a terribly stressful situation: birth.
Armed with just a little knowledge, you can catch this problem early and correct it before any damage is done. Obviously there is not time to wait for help to arrive. So what do you do?
Do you push the pregnancy back in so that it doesn’t come away from the uterus any farther? Absolutely not. The power of the uterine contractions would be impossible to overcome, and your efforts would only result in the mare pushing even harder.
Do you grab the exposed membranes and pull the pregnancy out so that the foal can “air breathe?” This would also be virtually impossible, and it would result in significant damage to the mare’s birth canal. Furthermore, by the time you succeeded in extracting the foal it would probably be dead. Remember, you’d not only be pulling the weight of the foal but also up to 75 pounds of liquid and placental membrane through a canal that is barely large enough to admit the foal alone.
The solution to the problem is very simple. First, be there when the mare foals. Watch for the water breaking. Whenever she lies down, walk around to a position where you can see her vulva. If you see the allantois-chorion at the vulva (remember what it looks like!), use whatever is handy–a knife, a pair of scissors, your fingers, whatever–and break it open. The allantoic fluid (“water”) will come out through the tear you’ve created, and the birthing will proceed. It’s that simple. By simply being there and knowing what to watch for, you can save the day with this simple procedure."
