The loss of one fetus in a multiple gestation pregnancy is a complex and emotionally challenging event. When this occurs around the 20-week gestation mark, the pregnancy shifts focus to the surviving twin, initiating a series of medical considerations. This mid-trimester timing means the physical outcomes and subsequent risks differ significantly from losses occurring earlier. Medical attention must immediately pivot toward mitigating potential complications for the remaining fetus and the mother.
Identifying the Fetal Demise
A single intrauterine fetal death is confirmed through a detailed ultrasound examination. The imaging study shows the twin with an absent heartbeat and lack of movement. Once the diagnosis is made, determining the type of placentation is necessary, as this factor profoundly influences management and prognosis.
The two main types of placentation are dichorionic (DC), where each fetus has a separate placenta, and monochorionic (MC), where the twins share a single placenta. This distinction is often established during earlier prenatal screening but becomes important following a loss. Dichorionic pregnancies generally face lower risks for the surviving twin. Conversely, monochorionic pregnancies, due to their shared placental circulation, face substantially higher and more immediate risks.
Immediate Physical Consequences
When a twin demise occurs at 20 weeks, the non-viable fetus is generally not expelled from the uterus. The body of the deceased twin remains in utero until the delivery of the surviving twin. The fetus undergoes a physical transformation rather than complete resorption, which is common with very early losses.
The most common physical outcome is the formation of a fetus papyraceus. This involves the dead fetal tissue losing fluid and becoming compressed and flattened between the uterine wall and the growing co-twin. The remains take on a mummified, parchment-like appearance.
The presence of the deceased twin does not typically cause infection or toxicity to the mother because the amniotic sac and membranes usually remain intact. The remains of the fetus papyraceus are delivered along with the placenta and the surviving twin at birth. This retention allows the pregnancy to continue, giving the surviving fetus the opportunity to reach full-term development.
Assessing Risks to the Co-Twin
The primary concern following a mid-trimester single fetal demise is the health and survival of the remaining twin, which depends entirely on chorionicity. In dichorionic pregnancies, the two fetuses have separate placental and vascular systems. The demise of one twin usually does not cause an acute, systemic disturbance to the survivor, resulting in a co-twin death risk of approximately four percent.
Risks for a dichorionic survivor are secondary effects, such as increased likelihood of preterm labor or restricted growth. The presence of the dead fetus can sometimes trigger uterine irritability, raising the risk of early delivery, with the preterm delivery rate being around 57%. However, the neurological risk to the surviving twin in a dichorionic pregnancy is low, reported to be only about one percent.
The prognosis is markedly different for monochorionic pregnancies, where the fetuses share a single placenta and interconnected blood vessels. When one monochorionic twin dies, the sudden drop in blood pressure in the deceased twin’s circulation causes blood to rapidly shift from the surviving twin into the dead twin’s vessels through these shared connections. This rapid, unbalanced blood flow can lead to acute hypovolemia—a severe drop in blood volume—and shock in the survivor.
This acute blood loss and drop in pressure can result in severe ischemic injury to the surviving twin’s organs, particularly the brain. The lack of sufficient blood flow and oxygen can damage the white matter of the brain, leading to conditions such as periventricular leukomalacia or multicystic encephalomalacia. The risk of death for the monochorionic co-twin is significantly higher, at approximately 12 percent, and the risk of neurological abnormality is reported to be around 18 percent.
Management and Surveillance
Following the diagnosis, medical management focuses entirely on intensive surveillance of the surviving twin. For dichorionic pregnancies, management is generally expectant, meaning the pregnancy continues with close monitoring. Surveillance includes frequent ultrasound assessments to check the survivor’s growth and amniotic fluid volume.
In monochorionic pregnancies, surveillance must be more aggressive due to acute risks. Detailed Doppler studies, which measure blood flow in the fetal vessels, are performed regularly. Particular attention is paid to the middle cerebral artery peak systolic velocity (MCA PSV), as an elevated MCA PSV can indicate fetal anemia and ongoing hemodynamic compromise.
Delivery timing is individualized and depends heavily on the chorionicity and the stability of the surviving twin. If the dichorionic survivor remains stable with normal growth, the goal is often to continue the pregnancy until closer to term. For monochorionic pregnancies, the higher risk of sudden deterioration means a planned, early delivery may be considered once the fetus reaches an acceptable gestational age for lung maturity.
The medical team provides continuous information and support to the parents throughout this period. This ensures they are aware of the specific risks and the rationale behind the intensive monitoring and management decisions. The goal is to maximize the chances of a healthy outcome for the surviving twin while weighing the risks of remaining in utero against the risks of preterm delivery.