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Everything you ever wanted to know about your amniotic sac (and then some) 
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 this may be a preferable alternative to induction for those of you with care providers pushing for other 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.
21st-Feb-2008 07:47 pm (UTC)

This is an excellent post! Thank you so much for putting this up there.

Have you considered becoming a CBE? :)
21st-Feb-2008 08:34 pm (UTC)
Certified Babystuff Explainer? ;)
21st-Feb-2008 07:51 pm (UTC)
Wow, excellent post!
21st-Feb-2008 08:03 pm (UTC)
thank you so much for posting this.
21st-Feb-2008 08:14 pm (UTC)
Awesome post.
21st-Feb-2008 08:35 pm (UTC)
I almost included a line that said, "And then there's freaks like ever_abstract, who have extra waterbags for no good reason." I opted to be more tactful. ;)
22nd-Feb-2008 02:28 am (UTC)
Haha. I would have LOLd. :D

When I finally decide on a place to bury that sucka- I'll take a picture for you.
22nd-Feb-2008 02:32 am (UTC)
I still have Donovan's nearly 2 year old placenta in my freezer because I can't decide where to bury it!
22nd-Feb-2008 02:56 am (UTC)
yeah... this is becoming an issue for me too:p
21st-Feb-2008 09:11 pm (UTC)
Another disadvantage of amniotomy not mentioned in the article you linked to is that it can cause the baby to be malpositioned. The baby's job during birth is to turn and if the sac is ruptured before the baby is in the proper position it can make it more challenging for the baby to turn.

I've attended a few births where this happened, the sac was manual ruptured and the baby got stuck OT (occiput transverse-aka sideways). One ended up in a c-section because the baby was not able to turn into the proper position-this was with a lot of moving and repositioning on the mother's part. In the other two the doctors manually turned the baby's head so that it was able to progress down the birth canal, but the births were probably a lot longer than they would have been if they hadn't ruptured the membranes to "move things along".

(I know you know this, it's just something for others to think about)
21st-Feb-2008 09:31 pm (UTC)
An excellent point. I'm going to put another little note up in the article that references your comment, because I think it's worth mentioning.
21st-Feb-2008 10:05 pm (UTC)
yes! i had that happen in my birth and it led to an incredibly long pushing stage (4.5 hours), narrowly escaping further intervention. i really wish i had declined the stupid cervical check, in hindsight.
22nd-Feb-2008 04:34 pm (UTC)
I had been looking for studies a few days ago that showed thing and couldn't find them...do you have links?
22nd-Feb-2008 05:29 pm (UTC)
I don't think I have anything online. We talked about it in my doula training and I think it might be in some of my books. I can take a look when I get home if you want.
22nd-Feb-2008 06:40 pm (UTC)
I would really appreciate it :) I have seen it stated here and there, and it makes sense, but so do a lot of birth assumptios that turn out to not be true (like continuous monitering -> better outcomes :-p)
24th-Feb-2008 04:55 pm (UTC)
From Pregnancy, Childbirth and the Newborn by Simkin, et al, pg 267
If the baby's head is malpositioned, intact membranes may provide some cushioning and a little room for the baby's head to wiggle into a more favorable position. Removing the cushion by rupturing the membranes sometimes fixes the baby's head and impairs these subtle movements, possibly causing a longer or more painful labor.
She cites the Cochrane Review as her source for this info.

In Sheila Kitzinger's The Complete Book of Pregnancy and Childbirth, she cites the following as a risk of amniotomy (p 330):
HEAD MOLDING Some obstetricians are concerned about head molding and disalignment of the cranial bones, which may be increased after amniotomy. There is disagreement about this.
21st-Feb-2008 10:35 pm (UTC)
That was very informative in a non scary way. I'm just a bit worried the way they seem to send you home if your waters haven't broken. When I was born I came with the water, gave my mom and the nurse a fright. My siblings were the 'normal' water breaks first birth.
21st-Feb-2008 11:17 pm (UTC)
Well, giving birth suddenly isn't the worst thing in the world. ;) My second-born (a planned, unassisted home birth)'s head came out when the water broke, and my friend's baby was born with the bag of waters still intact. Having the water break as you are actively pushing is really common, so don't worry if yours don't. The broken water, in and of itself, tells you very little about your birthing timeline!
22nd-Feb-2008 12:48 am (UTC)
Great detailed information! Thanks! I love learning more about the science of pregnancy.
22nd-Feb-2008 01:18 am (UTC)
This was a FASCINATING summary, thank you very much for taking the time to explain this. I'm 39 weeks along and I had no idea there were two stages.

Thank you!
22nd-Feb-2008 01:48 am (UTC)
It's likely that your waters (both bags at once) will break in one big gush at some point in labor, but if you spring a little leak instead, remember that's normal, too! I've had one time of slowly leaking waters (followed by amniotomy, which I would have declined, had I been better education) that happened nearly 12 hours before my baby was born and one time of a great big GUSH that was accompanied by my second baby's emerging head!
22nd-Feb-2008 03:00 am (UTC)
Thank you so much for posting something so accurate and informative!
23rd-Feb-2008 04:12 pm (UTC)
Also, amniotomy does not speed spontaneous labor or improve outcomes according to the linked article. Might be worth adding :)
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