Unequal Placental Sharing

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What is unequal placental sharing?

Unequal placental sharing is a condition that is particular to Twin Pregnancies that share a placenta, otherwise known as Monochorionic Twin Gestations. In monochorionic twins, a single placenta that was designed originally for 1 fetus is shared between 2 fetuses. Each twin receives a portion of the blood supply from the placenta. The ideal situation is one in which each baby receives half of the blood supply and therefore each will receive half the amount of nutrients and oxygen supply that they require for growth. In most cases, one baby receives 40 % and the other 60% and this usually sufficient for normal growth. Unequal placental sharing may be defined as 1 twin receiving blood from > 60% of the placenta. This may occur in 15% of twin pregnancies that share a placenta.  This unequal sharing is largely affected by where the umbilical cord of each twin inserts in the placenta. It leads to one twin being significantly smaller or growth restricted in severe cases.

How does this condition present?

Unequal placental sharing may be detected on an ultrasound that is done at any time in the pregnancy usually after the second trimester.  A difference in the weight of the fetuses is noted.  Usually a difference of greater than 20% is considered abnormal. Additionally, a difference in the amount of amniotic fluid between the two sacs may be seen. One baby has a normal pocket of fluid while the other has too little fluid (Oligohydramnios). This condition is very often confused with Twin to Twin Transfusion Syndrome (TTTS) in which there is also a discrepancy in the amount of fluid in each sac however; the one of the twins has an excess amount of fluid (Polyhdramnios) while the other has Oligohydramnios.  TTTS may co-exist with unequal placental sharing. 

What are the risks to the babies?

Birth weight discordance has been associated with increased risk of complications including  newborn death, low Apgar scores (test of physical condition and need for extra care), newborn intensive care unit admission, newborn oxygen requirement, and jaundice. These babies are also at risk of all the complications associated with prematurity (Respiratory distress syndrome, necrotizing enterocolitis, intraventricular haemorrhage).

Is there any treatment for the fetuses? 

Unfortunately there is no treatment for unequal placental sharing as the location of the insertion of the umbilical cord and the portion of placenta that supplies each fetus cannot be changed. If this condition is diagnosed prior to viability i.e. a gestational age that the fetus is not able to survive, there is the option of radio-frequency ablation (RFA) to eliminate the blood flow to one fetus in order to improve the likelihood of survival of the other “normal” twin. This is done in order to prevent deleterious effects on the normal twin in the event that the growth restricted twin dies in utero.

Continued surveillance is done to by ultrasound to observe for growth and blood flow patterns within the umbilical cord and other fetal vessels, particularly in the smaller twin to detect increasing severity and plan intervention. Non-stress test may also be done to detect abnormal fetal heart rate patterns. Often this level of surveillance involves admission to hospital and administration of steroid injections to Mom to reduce the risks of prematurity.

How and when will delivery occur?

Delivery is usually indicated in pregnancies that share a placenta by 36 to 37 weeks gestation, which is if spontaneous preterm labour does not ensue by then.  If the complications of unequal placental sharing arise, delivery may occur at a very preterm (28 to 32 weeks) or extremely preterm (less than 28 weeks ) gestational age. This should also occur in a centre that can take care of babies this young.

References

Fick, A. L., Feldstein, V. a, Norton, M. E., Wassel Fyr, C., Caughey, A. B., & Machin, G. a. (2006). Unequal placental sharing and birth weight discordance in monochorionic diamniotic twins. American Journal of Obstetrics and Gynecology, 195(1), 178–83. doi:10.1016/j.ajog.2006.01.015

Machin, G. A. (1997). Velamentous cord insertion in monochorionic twin gestation. An added risk factor. J Reprod Med, 42(12), 785–789.

 

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