The visual presence of two separate fluid-filled structures within the uterus during an early ultrasound often suggests a multiple gestation. However, the meaning of two sacs is not always straightforward and requires careful medical interpretation. Healthcare providers perform follow-up scans and tests to confirm exactly what these structures represent and to monitor the development of the pregnancy.
Defining the Gestational Sac and Early Development
The gestational sac is the first structure visible on an ultrasound that confirms a pregnancy is located within the uterus. This small, dark, round or oval fluid collection appears on a transvaginal ultrasound at approximately 4.5 to 5 weeks of gestation. It is surrounded by a white, reflective rim of tissue, which represents the forming placenta and the membranes.
The sac contains the developing embryo and the amniotic fluid that cushions and protects it. Around 5.5 weeks, a smaller, circular structure called the yolk sac becomes visible inside the gestational sac, providing initial nourishment to the embryo. The visualization of the yolk sac confirms a definite intrauterine pregnancy, while the embryo itself is typically visible around six weeks.
The Primary Significance: Twin Pregnancies
The most common meaning of seeing two distinct gestational sacs is a twin pregnancy, where each baby has its own separate environment. This specific setup is known as a Dichorionic-Diamniotic (Di/Di) pregnancy, indicating the presence of two chorions and two amnions. The chorion is the membrane that develops into the placenta, and the amnion is the sac containing the amniotic fluid.
Di/Di twins are the most frequent type, accounting for about 70% of all twin pregnancies, and carry the lowest risk profile of all multiple gestations. Because each baby has its own dedicated placenta and amniotic sac, they do not share resources. This separation significantly reduces the risk of complications arising from shared placental circulation.
All fraternal twins, resulting from two separate eggs fertilized by two separate sperm, are Di/Di twins. Identical twins, originating from a single fertilized egg that splits, can also be Di/Di if the split occurs very early (between one and four days after fertilization). The presence of two separate sacs with a thick dividing membrane on a first-trimester ultrasound is the characteristic finding for this lower-risk twin configuration.
Scenarios Where Two Sacs Do Not Equal Two Babies
While two sacs often indicate twins, other less common scenarios exist where a second sac-like structure is identified on an early ultrasound. One possibility is Vanishing Twin Syndrome (VTS), a phenomenon where one of two initially visible gestational sacs or embryos fails to progress and is subsequently reabsorbed by the mother’s body. This can be a common occurrence, with a reported prevalence of 15% to 35% in twin pregnancies, and it usually happens in the first trimester.
The loss of one twin is often attributed to developmental issues, such as chromosomal abnormalities in the non-surviving twin. If the loss occurs early in the first trimester, the prognosis for the surviving twin is excellent, and the mother rarely experiences lingering symptoms. The other sac simply vanishes from view on follow-up scans, leading to a singleton pregnancy.
A completely different scenario involves a Pseudogestational Sac, which is an accumulation of fluid or blood within the uterine cavity, not a true gestational sac. This structure can mimic a true sac on an ultrasound and is sometimes associated with an ectopic pregnancy, where the actual gestation is implanted outside the uterus. A pseudogestational sac often appears more elongated and is centrally located, lacking specific features like a yolk sac or the double decidual sign that characterize a true intrauterine pregnancy. Clinicians must use careful sonographic evaluation and serial blood tests to distinguish a true sac from a pseudogestational sac and rule out an ectopic pregnancy.