The gestational sac is a structure that surrounds an embryo, which is a baby in the very early stages of development. It encloses not only the embryo, but also the amniotic fluid, which helps to nourish and protect the developing baby. The gestational sac is the structure ultrasound technicians look for when they need to confirm the presence and viability of an early pregnancy. In fact, it may be the first pregnancy structure ultrasound technicians can see in early pregnancies.
In the early weeks of pregnancy, a woman may take a pregnancy test and miss a period. These things often serve as confirmation that a woman is pregnant. Unfortunately, they are not a 100% reliable confirmation. A woman may miss a period for a number of reasons, and pregnancy tests can be wrong. For example, a woman may have a chemical pregnancy, which means an egg is fertilized but does not implant in the woman's uterus. To confirm a healthy pregnancy that is progressing as it should, some obstetricians order early ultrasounds.
Generally, the gestational sac is visible around the fifth week of pregnancy. Obstetricians often use gestational dates for pregnancy, which means a woman is five weeks pregnant when five weeks have past since her last menstrual period. In reality, her baby may be only three weeks into development by this time. The detection of a gestational sac at this stage is a positive sign. It is not a guarantee the pregnancy is healthy and will proceed, however.
Shortly after the sac becomes visible, an ultrasound technician may be able to detect a yolk sac within it. The yolk sac may be detectable as early as the middle of the fifth week following a pregnant woman’s last menstrual period. The detection of this structure is an important milestone, as the yolk sac is the developing embryo’s first nutritional source. As with the gestational sac, its visibility does not guarantee a healthy pregnancy. Its absence after the sixth week of pregnancy, however, may be a cause for serious concern.
In some cases, ultrasound images may show characteristics of the sac that indicate problems with a developing pregnancy. It may not grow as expected, for example. After the fifth week, it should grow about 0.0394 inches (1 millimeter) per day until until around the ninth week of pregnancy. A gestational sac may be shaped abnormally or even appear to be empty long after the yolk sac should be visible.