Cause of sIUGR in twins
sIUGR can affect dichorionic (separate placentas and amniotic sacs) or monochorionic (a single, shared placenta) twin gestations but is more common in monochorionic twins. For both monochorionic and dichorionic twins, it can be associated with a fetal anomaly in one twin or a velamentous cord insertion (where the umbilical cord inserts into the membranes instead of directly into the placenta). For dichorionic twins, it can also be due to one twin having a better location for the placental implantation.
How common is sIUGR?
sIUGR affects 10–15% of monochorionic diamniotic twin pregnancies and has several additional possible causes in these types of twins. The most common cause is unequal placental sharing. In addition to a fetal anomaly or velamentous cord insertion, unequal distribution of cells when the identical twins initially split are an additional cause. There are three subtypes of monochorionic sIUGR pregnancies that have been described. These pregnancies are at increased risk of twin-to-twin transfusion syndrome.
Diagnosis of sIUGR
The diagnosis is made by ultrasound using an estimate of the fetal weight or the size of the abdomen to determine if one of the fetuses is smaller than expected. Ultrasound is usually recommended to be done regularly during a twin pregnancy to assess for this condition.
Types of sIUGR
sIUGR is divided into three separate types, based on blood flow in the umbilical artery as measured by Doppler ultrasound. A given pregnancy may shift from one type to another during the pregnancy.
Type 1 – consistent forward flow in the umbilical artery Dopplers. This has the best prognosis and is associated with the highest gestational age at delivery.
Type 2 – absent or reverse flow noted in the umbilical artery Dopplers for the IUGR fetus.
Type 3 – intermittent absent or reverse flow noted in the umbilical artery Dopplers for the sIUGR fetus. This is unpredictable and is associated with a higher rate of neuroimpairment.
sIUGR prognosis
For both dichorionic and monochorionic twins, the prognosis is determined by the degree of discordance and the presence or absence of abnormalities in the blood flow to the fetuses or within blood vessels in the fetus (Doppler measurements) as this will predict the gestational age at delivery. All twin pregnancies are at increased risk of preterm birth and for stillbirth or intrauterine fetal demise. Due to these risks, they require very close surveillance. They may benefit from frequent monitoring, including ultrasounds and prenatal testing to assess prognosis and optimize the timing of delivery. For some cases of sIUGR, there may be a role for fetal interventions.
Treatment of sIUGR
Unfortunately, there is no treatment to improve growth or to prevent the development of sIUGR. Making the diagnosis allows potential treatments to improve the outcomes if early delivery is required and also allows optimization of the timing of the delivery.
Why choose Michigan Medicine?
The Michigan Medicine Fetal Diagnosis and Treatment Center has extensive experience caring for all types of multiple gestations. Our maternal fetal medicine physicians provide the highest level of prenatal assessment and delivery planning and our neonatology physicians provide neonatal intensive care for those pregnancies that do require preterm delivery.
Make an appointment
To make an appointment, please call 734-763-6295.