Unequal placental sharing
Twin-to-twin transfusion syndrome (TTTS) is one of the most common conditions affecting monochorionic twins, identical twins who share one placenta. However, not all monochorionic twin pregnancies with unequal fluid levels or different-sized twins have TTTS. These characteristics can also be seen in a condition called unequal placental sharing.
When two fetuses share one placenta, their umbilical cords may implant anywhere — there is no set or predictable pattern. Depending on where they implant, one twin may get a smaller share of the placenta than the other, resulting in less blood flow and nutrition to one fetus, and more to the other. This is called unequal placental sharing.
As a result, the twins may grow differently. The "normal" twin typically has normal or generous, but not excessive, amniotic fluid level. The smaller twin may have either normal amniotic fluid volume or, if its growth becomes progressively restricted, can develop low fluid levels, called oligohydramnios.
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Doctors use ultrasound to determine the severity of unequal placental sharing. The greater the difference in size and weight between the twins, and the lower the fluid levels for the smaller twin, the more serious the situation.
Doctors also monitor the blood flow in the twins' umbilical cords using Doppler ultrasound.
Unequal Placental Sharing Versus TTTS
It can be challenging to differentiate unequal placental sharing from TTTS. Many TTTS pregnancies have some element of unequal sharing, and many pregnancies with unequal sharing may have some element of TTTS. However, when unequal sharing is the more significant aspect of the problem, amniotic fluid discrepancies typically don't reach the levels seen with severe TTTS, and the issue is more about the size and weight discrepancy.
TTTS is defined by having a deepest vertical pocket (DVP) of 8 centimeters or greater in the recipient twin's sac, with a DVP of 2 centimeters or less in the donor twin's sac. With unequal placental sharing, the fluid for each twin may be normal, or the smaller twin may exhibit some degree of low fluid related to its restricted growth. The difference in size between the twins may be marked and may reach 40 percent or greater. A difference of up to 20 percent is considered within the normal range for monochorionic twins.
Twins with unequal placental sharing have to be followed very closely for possible development of TTTS. Ultrasound exams are performed regularly to calculate the twins' weights and watch their growth and fluid levels. Although the smaller twin's growth may be somewhat restricted, in most cases the smaller twin will grow well enough to function normally.
The goal is to get the smaller twin safely to a gestational age when early delivery is an acceptable alternative to continued or worsening growth restriction in the womb, while remaining mindful of the other twin's best interests.
Vascular Connections Between the Twins
Virtually all monochorionic twins have vascular connections within the single shared placenta. Because there are different kinds of blood vessels, there are different kinds of connections: an artery may connect to an artery, a vein to a vein, or an artery to a vein.
In TTTS, the culprit is an abnormal connection between an artery from one twin and a vein from the other. Arteries have the ability to pump blood in one direction, while veins accept whatever blood is pumped into them. Therefore, when an artery from one twin connects to a vein in the other, blood flows from the powerful artery into the vein, leading to a net transfusion from one twin to the other.
In some cases, an artery from one twin connects with an artery from the other twin. Both vessels are powerful and can pump blood in either direction, so in these cases, blood typically flows back and forth between the twin's circulations, rather than exclusively in one direction. This type of connection seems to offer a protective effect, and in some cases will balance out the worrisome one-way flow that an artery-vein connection can cause.
We are now able to look for these types of connections using ultrasound. We tend to find arterial-arterial connections more often when there is unequal placental sharing rather than true TTTS, which might help explain why these twins are at less risk. In these cases, the main concern is the size and weight discrepancy, and specifically, determining the critical point when the smaller twin's share of placenta will no longer be enough to safely allow it to grow and thrive in the uterus.
There are no treatments for unequal placental sharing, because the amount of placental share for each twin cannot be changed. Amnioreduction and laser treatment are not beneficial, and could even make things worse, as the underlying problem is not transfusion between the twins.
Instead, the smaller twin is monitored very carefully for signs of difficulty. Depending on the stage of the pregnancy, this may lead to hospitalization for closer monitoring or delivery. It is very important to keep close watch on the smaller twin — if the smaller twin is stillborn, it can possibly cause serious problems for the surviving twin, due to the vascular connections between them.
In severe, early cases of unequal sharing, in which the smaller twin is severely growth restricted and at significant risk of dying before reaching a viable gestational age, radio-frequency ablation (RFA) of umbilical cord flow to that twin may be an option, in order to protect the normally grown twin from the possible adverse effects of the other twin's stillbirth.
UCSF closely monitors patients with unequal placental sharing using a combination of ultrasound and non-stress tests up to three times a week, and in some cases, continuous monitoring in the hospital. In this way, we maximize the benefits of longer gestation and maturity of the fetuses, while maintaining close surveillance of the smaller twin for signs that delivery is necessary.
UCSF Benioff Children's Hospitals medical specialists have reviewed this information. It is for educational purposes only and is not intended to replace the advice of your child's doctor or other health care provider. We encourage you to discuss any questions or concerns you may have with your child's provider.
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