What is selective Intrauterine Growth Restriction (IUGR)?
Monochorionic twins are twins that share the same placenta – this means that there is one placenta for two fetuses. Each fetus is connected to its side of the placenta (by its umbilical cord). In addition, vessels on the surface of the placenta usually connect both twins’ circulations. Monochorionic twins have the highest risk of complications because the well-being of one fetus crucially depends on that of the other. One of the possible complications is known as selective Intrauterine Growth Restriction (sIUGR).
Selective Intrauterine Growth Restriction (sIUGR) means that one identical twin is of normal size while the co-twin is significantly smaller. sIUGR occurs in 10% to 15% of monochorionic twins.
The main cause for the development of sIUGR in identical twins is unequal placental sharing. Although each twin should utilize approximately half of the placenta, in some cases, one twin has a very small portion – and therefore a smaller supply of nutrients and a harder time getting enough oxygen. That fetus may therefore be significantly smaller (growth-restricted) than the co-twin and could be at risk for stress and distress.
sIUGR is associated with a substantial risk of fetal and newborn complications for both twins, and an increased risk of intrauterine fetal death (IUFD). The death of one identical twin is reported to result in the death of the larger twin in 10-15%. Further research has shown brain damage in 15-20% of surviving monochorionic newborns. Even if both twins survive, sIUGR is associated with substantial risks for the one that is growing normally.
How is it Diagnosed?
The diagnosis of sIUGR is established by ultrasound. A monochorionic (single placenta) twin pregnancy is first confirmed. The weight of each twin is estimated from various ultrasound measurements (estimated fetal weight, or EFW). A fetus is said to be growth-restricted if these measurements suggest a fetal weight that is far below the expected norm. Close follow-up becomes necessary to rule out worsening of the process and the twins are checked regularly for signs of distress.
The management strategy for the twin in distress with sIUGR remains a challenge. It may include close monitoring, fetal therapy, or early delivery if severe deterioration is seen in the sIUGR twin, and prolonging the pregnancy is felt to be more dangerous than delivering the twins prematurely.
Expectant management is an option if the growth-restricted twin is not in distress. Serial follow-up exams are performed to evaluate the fetuses’ condition.
Elective delivery of the twins early in pregnancy may pose a separate set of problems. When the fetuses are delivered prematurely, they may develop neurological, respiratory and other disorders. The risk of newborn death is also increased. Thus, delivery for the small twin who is in distress exposes the normal twin to prematurity and its complications.
If progressive sIUGR is noted earlier in pregnancy, when fetuses are too immature to be able to survive outside the womb, premature delivery is not an option. Unfortunately, there is no available treatment for the sIUGR twin. However, prenatal intervention may be required if the smaller twin is so sick that demise is imminent, as its death may seriously affect the health of the other twin: the surviving twin may bleed in to the smaller twin through the connecting blood vessels of the placenta.
Fetal therapy may involve two procedures: cord occlusion or fetoscopic placental laser coagulation.
1. Umbilical cord occlusion of the sIUGR twin: This is only done when it is believed that the sIUGR twin is at such a great risk of dying soon, that it is preferable to sacrifice it in order to save the healthy one. By blocking off the umbilical cord of the small sIUGR twin, the normally growing twin is protected from bleeding out in its co-twin. This procedure is performed with either a special forceps or a specialized needle (called a radiofrequency ablation needle) to stop the blood flow through the cord of the targeted fetus. The growth-restricted fetus dies and remains inside the uterus for the remainder of the pregnancy. It is the most straightforward procedure, and poses the least amount of risk to the normal twin, which has an expected survival of 80% to 90%.
2. Fetoscopic placenta laser coagulation: This procedure is not often recommended for sIUGR. It is the technique that is used in severe twin-to-twin transfusion syndrome (TTTS), when BOTH fetuses are sick. This surgical approach utilizes an operative fetoscope to deliver laser energy that burns off the connecting blood vessels found on the surface of the common placenta. Once the vascular connections between the two fetuses become sealed, no further blood exchange can take place between them. This technique is very effective in TTTS, because both fetuses are at risk of dying. Fetoscopic placenta laser coagulation carries a higher obstetric risk than a needle radiofrequency ablation (see above), and is therefore associated with a higher risk to the healthy twin. On the other hand, it does not automatically kill the sIUGR twin.
Fetal therapy procedures should only be performed by surgeons with experience in fetoscopic operations in pregnancy. The final decision about choice of therapy will be influenced by the severity of fetal growth restriction and technical issues, such as the gestational age of the pregnancy, along with the parents’ preferences.
Most Fetal Treatment Centers have useful information online. A list of NAFTNet centers, links and their respective website can be found by clicking here.
Other useful sites, including national organizations, patient- and parent support groups, professional societies and governmental agencies, can be found on our resources page. Be aware that, by clicking on these links, you will leave the NAFTNet site. NAFTNet does not endorse the content of these individual web sites.
Below are a few sites that have specific information on IUGR:
Images of sIUGR
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