An early pregnancy ultrasound often reveals structures that spark immediate questions, especially when more than one is identified within the gestational sac. The appearance of multiple sacs so early in development is a significant finding that prompts investigation into the potential for a multiple gestation pregnancy. Understanding the meaning of these early sonographic signs requires examining the temporary structures supporting the embryo. This information helps clarify what the initial images suggest about the number of fetuses and the pregnancy type.
The Purpose of the Early Yolk Sac
The yolk sac is one of the first structures visible on an ultrasound, often appearing around five to six weeks of gestation. It forms within the gestational sac and serves as the embryo’s primary support system during the earliest stages of development. The yolk sac’s presence and normal appearance are important indicators of a healthy, ongoing pregnancy.
The human yolk sac provides initial nourishment to the developing embryo before the placenta is fully formed and functional. It is also the first site of blood cell production, a process known as hematopoiesis. This structure also helps in the circulation of gases and the formation of cells that will eventually contribute to the umbilical cord and the digestive tract. The yolk sac is a transient structure that typically regresses and disappears as the placenta takes over its functions later in the first trimester.
The Direct Implication of Two Yolk Sacs
The detection of two separate yolk sacs in an early ultrasound is a strong indicator of a multiple gestation pregnancy. Each yolk sac is dedicated to supporting a single developing embryo, meaning two sacs almost always signal the presence of two separate embryos. This finding helps medical providers classify the pregnancy as dichorionic/diamniotic (Di/Di), which is the least complex type of twin pregnancy.
Dichorionic/diamniotic means each twin has its own separate outer membrane (chorion) and inner fluid-filled sac (amnion). Since the chorion develops into the placenta, this configuration typically means each twin has its own placenta, offering a more independent environment for growth. All fraternal twins are Di/Di, but about one-third of identical twins also have this arrangement if the fertilized egg splits very early.
A Di/Di pregnancy generally carries a lower risk profile compared to twins who share a placenta or amniotic sac. The definitive confirmation of two separate pregnancies, however, requires a follow-up ultrasound to identify two separate fetal poles, which are the visible embryos themselves, shortly after the yolk sacs are seen. Finding two distinct fetal heartbeats solidifies the diagnosis of a twin pregnancy.
Navigating the Possibility of a Single Fetus
While seeing two yolk sacs is an initial sign of two pregnancies, it does not guarantee two live births. A phenomenon known as Vanishing Twin Syndrome (VTS) occurs when one of the embryos fails to develop or is lost early in the first trimester. The tissue from the non-viable embryo or sac is typically absorbed by the surviving twin or the parent’s body, often without symptoms.
VTS is a common occurrence, with estimates suggesting it happens in 15% to 35% of pregnancies that start as a multiple gestation and are detected early by ultrasound. The early loss of one twin is generally considered to pose minimal to no risk to the surviving fetus, especially if it happens before the second trimester. The surviving fetus continues to develop as a singleton pregnancy.
The possibility of VTS is why healthcare providers rely on sequential ultrasound monitoring to confirm the viability of each embryo over time. If a second yolk sac is seen but a corresponding fetal pole or heartbeat never develops, or if a heartbeat is detected and then stops, VTS is the likely explanation. This scenario explains how a pregnancy that initially showed two supporting structures can ultimately result in a single birth.