The term vanishing twin syndrome describes the spontaneous loss (demise) of one fetus in a set of twins or multiples in the uterus. Spontaneous loss of a twin most often occurs in the first trimester. The remaining fetus continues to grow normally, and the non-developing fetal tissue is partially or sometimes fully reabsorbed. This complication of pregnancy has become more frequently documented because of the increased use of early ultrasonography in multiple gestations.
The loss of one twin may occur in 20 to 30 percent of multiple gestations, although the exact frequency is difficult to ascertain. Vanishing twin syndrome is seen more often in women who have used assisted reproduction technology to conceive. The parents of a vanishing twin may experience grief, especially if the loss occurred later in the pregnancy, or they may feel relief that the remaining fetus or fetuses are still thriving and that the pregnancy can continue safely. Either way, you—the mother—can take solace in the fact that you did not cause the loss; it is a natural occurrence in some multiple gestations, and going from a twin pregnancy to a single pregnancy can greatly reduce the risk of complications.
Causes of Vanishing Twin Syndrome
In general, the cause of vanishing twin syndrome is unknown. Examination of the placenta after the remaining healthy twin is born may reveal some clues. Although many complications can lead to fetal death in utero, the most common reasons for a twin failing to develop seem to be malfunctions early in development and chromosomal abnormalities—factors that are out of the mother’s control. The surviving twin is usually normal. Another reason for loss of a twin may be a problem with its placenta.
Symptoms of Vanishing Twin Syndrome
Vanishing twin syndrome usually causes no symptoms, but sometimes causes some vaginal bleeding. The bleeding may be the first indication that a twin has stopped developing. In some cases, the bleeding may occur later in the pregnancy, weeks or even months from when the vanishing twin was first detected. The bleeding is usually not accompanied by any cramping or other symptoms, but it can be quite heavy—and alarming for the mother. Some mothers experience symptoms similar to miscarriage, including mild cramping, bleeding, and pelvic pain. Symptoms typically last a few days. The bleeding also results in an ultrasound being performed, which sometimes is how the vanishing twin syndrome is diagnosed for the first time.
Vanishing Twin Syndrome Diagnosis
A vanishing twin is often detected before the mother even knows she is carrying twins. A diagnosis of vanishing twin syndrome is confirmed by an ultrasound examination that shows a gestational sac with specific embryonic parts that is not developing as it should, or by the lack of a heartbeat in one of the fetuses. A problem with one fetus may also be revealed by blood test results that show decreased hormone levels.
Vanishing Twin Syndrome Treatment
No treatment is required for vanishing twin syndrome that is detected early in the pregnancy. You can continue taking care of yourself the way you did before learning of the vanishing twin, and do not need to do anything differently. Your doctor may want to monitor the health and growth of your remaining twin more carefully and watch you closely for signs of complications. The remaining twin is generally not affected by the vanishing fetus. If a vanishing twin is detected early in your first trimester, there is a very good chance that you will have a normal pregnancy and deliver a healthy single baby.
How the pregnancy will be managed following the in utero demise of a fetus later in pregnancy depends on the gestational age and chorionicity of the surviving twin. The term chorionicity refers to whether the twin pregnancy is dichorionic, in which each twin has its own chorionic and amnionic sacs, or monochorionic, in which the twins share the same chorion (the outer membrane enclosing the embryos). If the pregnancy is dichorionic, then no intervention is required unless there is a fetal or maternal reason for delivery.
In a monochorionic pregnancy (identical twins), loss of one fetus may require immediate delivery of the remaining fetus if it is considered to be mature or near maturity, meaning that the baby’s lungs have developed enough so that he or she can survive outside the womb. The decision to proceed with delivery is made when the risk of neonatal complications after delivery is less than the risk of remaining in utero. If the decision to hold off on delivery is made, there will be increased monitoring of the surviving twin. Any evidence that the health of the baby is compromised warrants immediate delivery.
Another option is available is some large cities, that of selective fetal termination with cord occlusion. If there is an identical twin pregnancy that is at risk for one twin to pass away in utero, then using invasive fetal surgery, the cord of this twin can be occluded. In this case, the targeted twin passes away immediately but the other twin normally will be unaffected. This is a risky procedure, with up to a 5 percent loss rate for the non-targeted twin.
Vanishing Twin Syndrome Complications
The results of first-trimester prenatal screening may be affected by a vanishing twin. The difficulty in interpreting the screening results is even more pronounced in pregnancies conceived by in vitro fertilization (IVF) or intra-cytoplasmic sperm injection (ICSI). According to one study, “Prenatal screening and diagnosis of chromosomal abnormalities, especially Down’s syndrome in IVF pregnancies, are complicated by higher maternal age, a high multiple pregnancy rate, a high risk of a vanishing twin, and an increased risk of chromosomal abnormalities, particularly in pregnancies after ICSI” (Human Reproduction Update, 2012).
In contrast, another study looked at the impact of a ‘vanishing twin’ on first trimester combined biochemical and ultrasound screening in pregnancies conceived after IVF and intracytoplasmatic sperm injection. They found that women pregnant after ART, who are diagnosed with a vanished twin at early ultrasound in Weeks 8–9, can have first trimester combined screening for Down’s syndrome (the NT test) performed using the same risk calculations as in singleton ART pregnancies. (Human Reproduction, Vol.24, No.1 pp. 55–62, 2009).
When vanishing twin syndrome occurs early in the pregnancy, complete reabsorption of the fetus and gestational tissues is the most common outcome. Formation of a fetus papyraceus (a “mummified” or compressed fetus) usually occurs when the loss occurs further along in the pregnancy. The fetus papyraceus rarely may affect the remaining twin. In the rare event that a twin stops developing in the second or third trimester, the remaining fetus may be at increased risk for intrauterine growth restriction, and the mother may be at risk for preterm labor, infection, or hemorrhage. Some studies have reported that there may be a slightly higher risk of cerebral palsy in a child who shares a uterus with a vanishing twin (possibly due to an undiagnosed identical twin pregnancy).