Twin Terms Part III: Plumbing Arrangements

White line laying on top of baby's face is the amniotic membrane

We’re done with the simple nomenclature and off to the plumbing designations, which can be a little complicated, and has lots of lingo, which you probably won’t be familiar with the first time you hear it. Don’t worry though –you soon will be!
Dizygotic (fraternal) twins will always have all of their own structures within the womb, often referred to as ‘di-di’. Each has their own water filled play area called an amniotic sac. Each amniotic sac is contained within its own outer covering, or chorionic membrane. The chorion is synonymous with the placenta. So every fraternal twin will have its own placenta.
Monozygotic (identical) twins however, have several possible ‘womb-mate’ arrangements. Usually you will see this expressed or referred to with the lingo ‘mo-mo’ or ‘mo-di’. Those words can be translated so that the first ‘mo’ or ‘di’ refers to the twins’ chorionicity, and the second ‘mo’ or ‘di’ refers to the twin’s amniosity. (Note that is not animosity, which will certainly come into play around age 2 or so! LOL)
The chorion is a membrane that surrounds the amniotic sac (or sacs) and it is synonymous with the placenta. So if there is one chorion, there will be one placenta. If there are 2 chorions, each baby will have its own placenta. Monochorionic means that there is one chorionic membrane and that the babies will share a placenta. Sharing a placenta does generally put you at a higher risk for twin to twin transfusion syndrome where one twin gets more blood from the placenta than the other, causing variations in growth which pose a hazard to both twin’s health. But stress over it because there are many monochorionic multiples that do just fine! Your doctor will do lots of ultrasounds to monitor their growth and make sure they are growing at approximately the same rate.
The exception in this case is that there are cases where babies are dichorionic and have their own placentas, but as they grow, and as the placentas grow, they fuse together. Once the placentas fuse, it can be difficult to determine if your twins actually shared a placenta or each had their own which fused together. So, if you find out you are having twins at your 20 week ultrasound, and there only appears to be one placenta, you may not know the true chorionicity of your twins unless you have the placenta examined after their birth.
Amniosity refers to whether or not your babies have their own amniotic sacs. If your twins are monoamniotic, (‘mono’ meaning 1), they have only one amniotic sac and share a swimming pool together inside you. If they are di-amniotic, (‘di’ meaning 2) that means there are two sacs and each baby has it’s own pool of water which is separated from his siblings by a curtain-like membrane.
Unfortunately, sharing one amniotic sac does have its risks with the possibility of cord entanglement, which can. For this reason, it is normal for your physician to admit you to the hospital for constant monitoring of the babies at the point which they are viable (able to live outside the womb), which is around 20 weeks. The reasoning is that they want to monitor the babies for distress, so that at the first sign of trouble (likely resulting from cord entanglement) they could deliver the babies and save their lives. Note that this is not the same as bed rest that was indicated because of a risk for preterm labor. It is mostly for monitoring purposes. If your physician does NOT advise this, I would seek a second opinion.
The number of placentas and amniotic sacs can best be determined by ultrasound at around 12 weeks. At that time, the placentas and amniotic membrane are more highly visible than they are early in pregnancy when they are very small, or later in pregnancy when placentas can potentially grow together.
Now, if you are having triplets or more, you could have any combination of shared or individual placentas. So two of the babies could share a placenta and the third have its own, or all three share one placenta, or each of the three have their own placenta. Same thing with amniotic sacs – some babies could share a sac while others have their own.
Ok, if you fell asleep during that, you are probably not alone! So here is a brief review!
Fraternal Twins = Dizygotic twins (originating from 2 separate eggs and sperms) Fraternal twins will ALWAYS be ‘di-di’, meaning dichorionic and diamniotic having each their own placenta and amniotic sac.
Identical Twins = Monozygotic Twins (originating from the same egg & sperm which divided into 2!) Identical twins can be ‘mo-mo’, ‘mo-di’ or ‘di-di’. That means that even if your twins have their own placenta and their own amniotic sacs, they could still be identical and the only way for you to learn that they are NOT identical would be to rule out monozygosity with one of the following tests:
1.    Babies are different sexes, therefore not identical (duh!)
2.    Babies have different blood types, therefore not identical.
3.    Babies have a DNA test which compares their DNA and determines zygosity.
I hope you studied well..there may be a quiz later!