Circumvallate and other variations of Placental Structure - Placental Lakes, Bilobed placenta, Velamentous cord insertion, Marginal or Battledore cord insertion, Anterior, Posterior, Fundal Placement

By: Aunt Doula (02/06/2023)

Both sides of the placenta are vital to it's proper function, and when issues arise it may be with either side, and each complication below will describe which side of the placenta the issue affects. Another vital area of understanding the placenta and pregnancy anatomy is that the placenta is a part of your amniotic sac. If you imagine the placenta as a lid, and the chorion sac as a cup, the two come together to make one complete outer "bubble" around the fetus that is separate from the inner membrane (the amnion) which is connected to the fetus along with the umbilical cord, covering the fetal side of the placenta and enveloping the fetus in the sac that will fill with amniotic fluid.

In a normal situation, the chorion membrane extends from the edge of the placenta which seals the outer sac. There are many images even in medical text looks that depict ambiguous anatomy and confusing perimeters that don't convey the actual way things look and work. The amnion is the inner sac which the baby actually floats inside and projects out of the umbilical connection on the fetus covering the cord, the fetal side of the placenta, and surrounding the baby, slowly filling with amniotic fluid until at about 16 weeks when the amnion and chorion fuse together, which becomes referred to then as the amniotic sac. The sac continues to fill with fluid until it reaches approximately 1 quart of fluid and in the later 3rd trimester slowly reduces to a just over 3/4ths of a quart. I can't wholly recommend searching for real images of a birthed placenta - suffice to say that if you are the least bit squeamish you need to be prepared!

As mentioned before, in a normal and healthy pregnancy the placenta is a round or oval shaped organ that is on average 2 inches/5 cm in thickness and 10 inches/25cm in diameter that grows in the presence of a pregnancy and attaches to the endometrium lining of the uterus. In the situations below, there are abnormalities to the shape, adhesion, blood vessels or the sac (which is a separate but an interconnected structure.) Most of these situations listed are rare, and as with all topics on this sub, the goal is to empower you - not scare you. Understand that the chances of any one of these abnormalities occuring is very small. Knowledge is power, but if reading about the things that could go sideways is not helpful to you, you can pass reading about these complications unless they specifically apply to you! If you have been told you have a healthy placenta, you need not read on - placental issues that you should know about that could emerge as an emergency are listed in for when and if you are ready.

Circumvallate Placenta

In this abnormal presentation of a placenta, the fetal side is affected. The two membranes grow past their designated connection points (the chorion should stay at the edge of the placenta and the amnion should only cover the fetal side of the placenta in one layer.) This inappropriate growth inward over the fetal side of the placenta causes a thickened doubling back over the fetal side of the placenta that prevents proper gas exchange and waste disposal. Depending on severity, the inappropriate growth thickens greatly creating a tight band to form that exerts squeezing pressure on the entire placenta as well as restricts blood vessels across the surface and makes the total functioning space much smaller leading to a drop in placental function. A circumvallate placenta presentation is always varied and may be found on ultrasound. Some mild circumvallate situations will simply be monitored for any progression of the abnormality. In severe cases, there may be frequent ultrasounds and monitoring by a specialist to ensure that the fetus is getting sufficient blood flow to remain in utero until term. Usually, a circumvallate placenta is not severe, but if you have been told you have one it can be confusing to try an find any information on the internet! Knowing how far the circumvallate border has progressed on the fetal side and if there has been any blood flow impacts are the two most important questions to ask your provider and ensure that you know what the plan is going forward.

There is no cure for a circumvallate placenta, and management is careful monitoring and possible early delivery if the blood flow is compromised significantly.

A closely related placental variation is a circumarginate placenta, and this is where the connection of the gestational sac and amniotic sac are connected more inward to the edge rather than directly at the edge of the placenta. The primary difference between these two variations is there is no doubling back of the membranes, and the placenta is not pulled tightly as in circumvallate presentations. It is thought that circumarginate presentations may be a function of the placenta attempting to expand after the 12 week mark to improve blood flow availability to the fetus.

Placental Lakes

Placental lakes are a common finding on ultrasounds, often at the 20 week anatomy scan. These lakes may be on either side of the placenta, and appear like black pockets because they are pooling of blood behind or within the placenta. Small lakes, up to 2 or 3, is considered normal and is often not mentioned. If large or multiple (greater than 3) placental lakes are mentioned in your report or by your doctor, it may come along with a notation of a thickened placenta and the possibility of placenta accreta may be investigated. Placental lakes on their own have not been associated with any increase in poor fetal or pregnancy outcomes, however if you are subsequently diagnosed with placenta accreta, there may be changes to your delivery which you can read about here

Bi-lobed/Tri-lobed placenta, placenta succenturiate

As the title suggests, it is possible to have a placenta that grows in two or three equally sized lobes. It is not well understood why this occurs, however there is usually enough function between all lobes to maintain a healthy pregnancy. What impacts a multi-lobed placenta pregnancy most is where the umbilical cord inserts into the placental tissue. It may connect in the center of one of the lobes or in the center between the lobes. Blood flow will have to be evaluated to ensure that adequate exchange of nutrients and waste is occuring to support the pregnancy.

Another possible placental presentation is placental succenturitate where one medium/regular sized lobe and one smaller "satellite" lobe are present. Just as with the bi/tri lobed placentas, it is of most importance where the umbilical cord is connected and if it can facilitate enough function to provide enough nutrients for the fetus. There is no treatment for these placental malformations, only careful monitoring.

Velamentous & Marginal/Battledore cord insertion and Vasa Previa complications

In an ideal and normal scenario, the umbilical cord is attached to the single large round/oval placenta in it's center, however for reasons we still do not fully understand, this may not happen. During the development of the structure that connects the fetus to the placenta, something occurs that ends up with the umbilical cord not attaching in the center of the placenta.

In a battledore insertion, the umbilical cord is connected to the placenta close to the edge instead of the center, this can compromise either side of the placental. This can lead to problems with adequate exchange of nutrients and waste, but often only needs to be monitored for issues. During delivery of the placenta it is important that the provider not apply any traction to the umbilical cord, as these connections are generally weaker at the edge and it may lead to the umbilical cord being pulled off the placenta - this is a significant complication and is one reason to consider expectant management in the third stage of delivery which you can read about here

In between a central (normal) insertion of the umbilical cord and the battledore (marginal/edge) insertion is the eccentric insertion which is like the second ring on a bullseye. Center, eccentric, battledore. Eccentric insertion has some influences to blood flow, but not as severely as battledore. If your provider informa you of an eccentric insertion it is still wise not to have traction applied during the third phase of delivery.

In velamentous cord insertion, for reasons again that we don't fully understand, the placenta end of the umbilical cord does not grow along the structure to connect it to the center of the placenta. Instead, the placenta end of the umbilical cord attaches to the amnion membrane and then the blood vessels within the umbilical cord grow unprotected between the chorion and amnion sacs to insert into the side of the placenta. The umbilical cord has a tough fibrous protective sheath covered in a substance known as Wharton's jelly to keep it slippery and un-knotted, that surrounds the three main blood vessels (sometimes two) that send blood to and from the fetus - this protective sheath ceases to grow with with vessels once it has embedded into the amnion and the vessels grow loose because they still need to find the placental connection. These vessels now are open to damage from the baby's movements, impacts to the abdomen and normal shifts of the sacs, and most importantly when the amniotic sac is ruptured at birth. If these vessels were to rupture before the baby had been born, catastrophic blood loss can occur. Velamentous cord insertion is a leading cause of miscarriage and it is due to the fragility of the unprotected vessels along with the compression of the vessels when the amnion and chorion fuse and the baby grows. It is exceptionally important that during delivery of the placenta if vaginal delivery has been approved that the provider does not put any pulling traction the umbilical cord to deliver the placenta which is described in the expectant management of third stage delivery here

A further complication in a velamentous cord complication is vasa previa - in this additional complexity, the placental end of the cord inserts into the amnion membrane and then the vessels pass under the fetus, across or near the cervix, and then connect to the placenta. This complication increases risks as any disturbances of the cervix can cause damage to the delicate and unprotected vessels adding in another risk factor for catastrophic blood vessel damage. Vasa previa can be monitored however it is a condition that will require an early delivery by C-section around 35 weeks in order to ensure that there will be no disturbance to these vessels by the cervix, the baby dropping into the pelvis and placing too much pressure on the vessels with their head, and to prevent blood loss if the amniotic sac is ruptured. See the drawings here to understand the velamentous and vasaprevia presentations - hand drawings, not photos.

Anterior, Posterior, Previa, Lateral/Side & Fundal Placenta Placement

During one of your ultrasounds, you should be told where your placenta is, relative to your uterus. You can ask if they don't mention it. This is usually not a big deal, or anything to worry about unless you have a previa, which can be read about here

• Anterior means your placenta in connected on the 'front' of your uterus, so as your belly grows out, your placenta is along that baby bump! An anterior placenta can complicate getting clear ultrasound pictures at times, as well as clear heart beat readings. This can usually be worked around by most providers, especially if you remind them that you have an anterior placenta. You may also not be able to feel kicks and movement from the baby as early as other pregnant people because the placenta absorbs some of the movement. Lastly, if you needed and amniocentesis or an ECV (external cephalic version) to rotate a breech baby, having an anterior placenta may make these a more complicated procedure.

• Posterior means your placenta is connected to the back of your uterus, so it is behind the baby. There are no big things to note about a posterior placenta, it's a pretty ideal position!

• Placenta previa is talked about at length but it means that your placenta has connected toward the bottom of your uterus and is completely or partially covering your cervix. This has its own dedicated page as it is an important topic.

• Lateral/Side means the placenta had attached to the left or right side of your uterus. This is another pretty decent connection place, and isn't associated with preventing procedures or making anything riskier during pregnancy and delivery.

• Fundal means the top of the uterus and this is where most placentas end up in the grand majority as it's the first place the embryo floats to upon exit of the fallopian tube. A fundal placenta is the most convenient placement for many reasons, and usually delivers well and is easiest to account for in a C-section.

No matter where your placenta is, knowing is an important piece of information in case you run into a situation and think "this monitor is right over my placenta" you can say something! Or if you are concerned about pain or just about anything going on with your pregnancy, being able to add this information in for a provider can speed up diagnostic processes by giving them as much information as possible.