The Twin Placenta

Twin Placentas and Fetal Membranes

Mother and two week old fraternal twins.
Ruth Jenkinson/Science Photo Library/Getty Images

The placenta is the body's only disposable organ. The placenta, with the attached membranes (amnion and chorion), are designed to function as the home, filter and feeding mechanism for pregnancy. In a twin pregnancy, there are many more variables for a twin placenta and the fetal membranes.

Fraternal twins, also known as dizygotic twins since they come from two eggs and two sperm, always have their own placenta and surrounding sacs. About 70% of monozygotic twins, or single egg and single sperm made identical twins, will end up sharing a single placenta which can cause potential complications. The most problematic is also the least likely (about 1%) and affects only identical twins, is the combination of a single placenta and a single sac.

Zygosity refers to how many eggs were involved with the conception of your twins. One egg (ova) makes monozygotic or identical twins, but two eggs are a set of dizygotic or fraternal twins. Chorionicity talks about how the membranes are laid out.

Chorionicity of Twin Membranes

Types of Twins
LifeART (and/or) MediClip image copyright 2008. Wolters Kluwer Health, Inc.- Lippincott Williams & Wilkins. All rights reserved.

The placenta has attached the fetal membranes, which consist of the amnion and the chorion. These two very thin layers are the fetal membranes and contain the baby and the amniotic fluid in a singleton pregnancy. In a multiple gestation you can have various combinations, including:

  • Diamniotic/Dichorionic (DiDi)
    This means that your babies have their own amnion and their own chorion. They are in separate membranes. This can happen with dizygotic (fraternal) twins as well as monozgotic (identical) twins. The placentas will be separate, but may fuse. When this happens in identical twins, it is determined by when the egg split. The sooner it splits the more likely you are to have separate placentas and membranes.
  • Monochorionic/Diamniotic (MoDi)
    These are more likely identical or monozygotic twins who share the outer layer, the chorion, but have individual inner sacs, the amnion. There are some dizygotic twins who will have this type of chorionicity.
  • Monochorionic/Monoamniotic (MoMo)
    This is the most rare, but potentially very dangerous situation where the babies share both the outer and inner sac, known as monoamniotic. The risks here are largely about cord entanglement and twin-to-twin transfusion syndrome (TTTS).

The following pages contain shots of actual twin placentas.

Healthy Twin Placenta

Twin Placenta
Photo © REW

In a singleton pregnancy, the placenta is designed to function for the duration of the pregnancy to support a single baby. At the time of birth, the placenta is roughly about a seventh the size of the baby, that's over a pound of placenta.

With a multiple pregnancy, you need twice as much placenta, be that two separate placentas or one larger placenta. You can see from this photo that this placenta is very large. It is from a 40 weeks long pregnancy for monozygotic twins.

Maternal Side of a Twin Placenta

Maternal Side Twin Placenta
Photo © REW

The maternal side of a twin placenta is lobular and looks like you'd imagine a brain would look. This is where the placenta connects to the mother's uterus. It is through this side that these connections help to feed the nutrients to the babies and filter the waste from the blood.

Fetal Side of a Twin Placenta

Fetal Side of a Twin Placenta
Photo © REW

The fetal side of the placenta is smooth. You can see the veins in the placenta, particularly near the umbilical cord on the right. You can also see two cords extending from the placenta. This doesn't tell you much about the fetal membranes without an exam.

The placental exam that occurs after birth will look for all of these items and more.

One Side of the Twin Placenta

One Side of the Twin Placenta
Photo © REW

As you can see, there is not a lot of space inside the fetal membranes of half this twin placenta. You can also see that there is only a thin layer separating the twins, even when there are both an amnion and chorion.

Separating the Amnion and Chorion in a Twin Placenta

Separating the Amnion and Chorion in a Twin Placenta
Photo © REW

You can see these thin membranes separate fairly easily during the placental exam. While they are easy to see through and separate, they are tough enough in pregnancy to withstand the fetal movement, kicks and twists of the twins. These fetal membranes are deceptively strong.

Two Twin Placentas Grown Together

Two Twin Placentas Grown Together
Photo © REW

When there are two placentas, they will sometimes grow together or fuse over the course of pregnancy. This is usually because they are competing for space on the inside of the uterus, which only has so much surface area. This can make it difficult to tell if there is one placenta or two that have grown together. This placenta you can see where there is a clear margin for one placenta and that they are only attached at the bottom portion.


Obstetrics: Normal and Problem Pregnancies. Gabbe, S, Niebyl, J, Simpson, JL. Fifth Edition.

Twin Pregnancy Complications. UCSF Fetal Treatment Center. Last Accessed 12/27/11:

Last Updated: 12/27/11

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