Unequal Placental Sharing

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.

Reviewed by health care specialists at UCSF Benioff Children's Hospital.

Related Information

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