Everything you ever wanted to know about your amniotic sac (and then some)

Everything you ever wanted to know about your amniotic sac (and then some)


I wrote this for the pregnant community today, but prasava said that I could share it here, as well. It's really geared towards women who have a more mainstream approach to their pregnancies (ie. follow a standard medical model of care), who may not know that amniotomy has risks involved or that having their waters break without starting labor doesn't automatically mean induction.

The topic of ruptured membranes, particularly having a "fore bag" of waters that breaks while an inner layers remains intact, has been brought up in several recent posts. I thought you might find this interested and helpful as you approach the end of your pregnancies.

Your baby isn't enclosed in a simple, single membranous layer. The "bag of waters" (amniotic sac) surrounding him/her/them is comprised of two layers: the chorion and the amnion. Other membranes have also surrounded your baby at different points in his/her development, and have helped with the development of the placenta and the umbilical cord.

The amnion is the innermost of the embryonic or fetal membranes, the sac in which the embryo is suspended. Early in the pregnancy, this membrane is in close contact with the developing embryo, but expands and moves away into a protective bubble as amniotic fluid production increases. This is your "inner bag" or "hind bag" of waters, the sac in which the baby is directly contained. When one is talking about the amniotic sac, this is what one is normally thinking of. For the sake of rupture of membranes, this is the one that really "counts" as having had your water break.

The chorion is the tougher outer membrane enclosing the embryo. It contributes to the development of the placenta. The chorionic villi (what is sampled in a CVS procedure) emerge from the chorion, invade the endometrium, and allow transfer of nutrients from maternal blood to fetal blood.

Identical twins may share a chorion or both a chorion AND an amnion, while fraternal twins each have their own chorion and amnion. In about 70% of identical twins, the twins will share a chorion (monochorionic), though some may also share an amnion (monoamniotic). Twins can share a placenta without being monoamniotic. The rare phenomenon known as twin-to-twin transfusion can sometimes occur in monoamniotic twins with a shared placenta, where twins receive an unequal amount of nutrition through the placenta.

Some women may actually have a true double bag of waters (two chorions and two amnions). This is uncommon and in some cases may be the result of a twin pregnancy where one twin was reabsorbed very early in the pregnancy, leaving one empty sac and one healthy developing baby.

The chorion and amnion are in contact with each other by the end of the pregnancy, often fused together so that when one breaks, so does the other, but they also can have a thin layer of amniotic fluid in between them. Many of you may have heard about women's bags of water "springing a leak" and then "resealing" -- in many of these cases, the chorion has developed a slight tear, allowing a minimal amount of fluid to escape. As the torn portions of membranes come into contact with each other again, however, they overlap and stick together, much like plastic wrap/cling film does when doubled over on itself. This is one reason why you will often see the more naturally-minded among us encouraging you not to rush to the hospital immediately, but wait to see if the fluid leakage stops.

This is one reason why women may experience a breaking of waters, sometimes one that seems pretty significant in its amount, only to be told by a doctor or nurse at the hospital that their waters are intact OR that only one layer has broken. I experienced this first hand, so I know the frustration of rushing to the hospital (as your care provider has told you) only to discover that you went much earlier than you needed to. Sometimes you will simply be sent home, but your care provider may also present the option of amniotomy (artificial rupture of membranes or AROM) to fully break your waters, especially if you are having contractions. Amniotomy has many disadvantages* and some advantages, and is a medical intervention you should research for yourself before the situation arises, to decide under what circumstances you would/would not feel comfortable having the procedure performed.

Remember that breaking waters (in a full term pregnancy) doesn't have to put you on a clock. There is no hard and fast rule regarding at which point the risk of infection increases, but the best way to avoid infection is simply not to insert anything into the vagina. If your water breaks, but contractions don't start right away, you might consider avoiding internal exams to check for dilation. Internal exams can introduce new bacteria/viruses into the vagina or push pre-existing bacteria/viruses further into the vagina. You always have a right to decline an internal exam, an induction, or any other procedure. If your labor has not started after a certain period of time after your waters have broken, ask your care provider about alternatives to induction, such as IV or oral antibiotics, or periodic monitoring of your health and your baby's. Your baby won't run out of amniotic fluid; it is constantly replenishing! The pockets of amniotic fluid can be measured via ultrasound; even for those of you who prefer to limit or avoid ultrasounds during pregnancy, this may be a preferable alternative to induction for those of you with care providers pushing for interventions.

I hope this hasn't been too boring and has answered some of your questions about the amniotic sac. I'm by no means a medical professional, but as an unassisted homebirther, I have made it my goal to learn as much about how my body works as possible, and to answer the many lingering questions I had about the many interventions performed (both with and without my consent) during my first birth (which was in a hospital).

*Amniotomy may also contribute to malpositioning of the baby, a disadvantage not listed in the link above.






Lilah Monger - lilah@ancientartmidwifery.com - AAMI Student Midwife #1769

New Student Advisor, Mentor Program Director, and Reading Room Coordinator