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Consider Placental Discrepancy in Discordant Twins


 

SAN FRANCISCO – Don't assume the presence of twin-twin transfusion syndrome if you see discordant sizes and weights in a monochorionic twin pregnancy; consider unequal placental sharing and other potential causes, Dr. Vickie A. Feldstein said.

Sometimes called “selective intrauterine growth restriction,” unequal placental sharing occurs when twins who share one placenta don't divide the placental resources equally so that one twin gets less circulation and nutrition.

“Twins in utero, like twins on the outside, don't necessarily share well,” she said at a meeting on antepartum and intrapartum management sponsored by the University of California, San Francisco.

Monochorionic twins typically get followed closely (at least every 2 weeks) in the last two trimesters because of the risk for twin-twin transfusion syndrome, which affects 15% of monochorionic twins and causes fetal death in up to 70% of cases if untreated.

Every ultrasound report from serial surveillance of monochorionic twins should include an estimate of percent weight discordance, said Dr. Feldstein, professor of clinical radiology and obstetrics, gynecology, and reproductive sciences at the university. To calculate percent weight discordance, take the estimated weight of the larger twin, subtract the estimated weight of the smaller twin, divide the sum by the weight of the larger twin, and multiply by 100.

If the discordance “is more than 15%, we're a little bit attentive,” she said. Discordance of 20% or more is cause for concern.

A 20% discordance may stabilize, with the twins following their own growth charts and ending up healthy but slightly different sizes. “This is one of the reasons I never refer to these as identical twins,” she noted.

If the discordance continues to increase, however, look for the cord insertion sites by ultrasound. With unequal placental sharing, typically the larger twin has a central cord insertion, and the smaller twin has a marginal cord insertion.

“If you do Dopplers of the cords of these twins, in general the twin with the better piece of placenta and central cord has normal umbilical artery Dopplers, and the smaller growth-restricted twin tends to have higher-resistance Dopplers from very early on,” Dr. Feldstein said.

If the smaller twin starts to fall off of its growth chart, early delivery may be in order.

All monochorionic twins have connections between arteries, veins, or an artery in one twin and a vein in the other. An artery in one twin draining into a vein of the other, bringing oxygenated blood to its sibling, is definitive for twin-twin transfusion syndrome. An artery-to-artery connection is protective for twin-twin transfusion syndrome. The difference affects how the patient should be counseled.

A diagnosis of twin-twin transfusion syndrome requires not just discordant sizes/weights but also the presence of polyhydramnios in one fetal sac and oligohydramnios in the other. The variability of twin-twin transfusion syndrome cases in age of onset, severity, acuity, and degree of discordance in size, weight, and amniotic fluid volumes can make it hard to recognize.

Unequal placental sharing and twin-twin transfusion syndrome can occur separately or concurrently. “I think of monochorionic placentas like snowflakes. They're all different, and anatomy is crucial,” she said.

If one twin has oligohydramnios and the other has normal amniotic fluid volume, it may be a case of unequal placental sharing, or there may be a more common problem such as a renal anomaly or rupture of membranes.

“We have a fair number of patients referred in for 'twin-twin transfusion syndrome,' and it turns out that one twin has ruptured membranes and there isn't twin-twin transfusion syndrome at all,” Dr. Mary E. Norton said in a joint presentation with Dr. Feldstein. “It's important to think about the common things that can cause oligohydramnios,” said Dr. Norton, director of perinatal medicine and genetics and professor of obstetrics, gynecology, and reproductive sciences at UCSF.

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