Dictionary DifficultyMiscarriageIrregular Gestational Sac Irregular Gestational Sac Reviewed by Dr. Temeka Zore OB/GYN, REI View our Editorial Policy Published: July 29, 2017 What Does Irregular Gestational Sac Mean? An irregular gestational sac is the sac of fluid in which an embryo develops in very early pregnancy that has irregular or ill-defined borders in comparison to a normal gestational sac. Gestational sacs can vary in shape to a certain degree, but an irregular gestational sac is usually a cause for concern as they are predictors of poor pregnancy outcomes. Ultrasounds that show irregular gestational sacs are normally followed up by further ultrasounds or other imaging to confirm the diagnosis and determine the risk of miscarriage. FertilitySmarts Explains Irregular Gestational Sac In early pregnancy, three structures can be visualized on a transvaginal ultrasound including the fetal pole/embryo that becomes the fetus, the yolk sac that nourishes embryo, and the gestational sac that encloses the embryo and yolk sac. These three structures can provide information regarding how the pregnancy is progressing. A gestational sac is the first structure visible on ultrasound imaging as early as 4.5 weeks. When a gestational sac is visualized it can: Be measured with a calculation called the mean sac diameter (MSD) to assess the development and estimate the gestational age. Provide additional information about how the pregnancy is progressing based on the shape of the structure. The appearance of a normal gestational sac: Is round or elliptical Is located in the upper body of the uterus called the fundus Contains a smaller sac called the yolk sac Contains the amniotic sac (which will eventually take up the entire sac) The appearance of an irregular gestational sac: Is generally larger than usual Has a position low in the uterus Has no embryo or yolk sac inside (with aMSD of 25 mm) Has a distorted shape Has thin walls A gestational sac with one or more of these features may indicate that the pregnancy is not viable with the potential to end in spontaneous miscarriage. Additional Concerning Factors Additional factors associated with poor prognosis at this early stage of pregnancy include: Discrepancy between the age and size (MSD) of the embryo. Abnormal fetal heart rate. Around 6 weeks gestational age, a typical fetal heart rate is 100-120 beats per minute (BPM). Share This Term> Related Terms Mean Sac Diameter Viable Pregnancy Transvaginal Ultrasound Yolk Sac Gestational Sac Fetal Pole Related Reading Best Books to Read After a Miscarriage Excerpt: The Other Half – What About Men and Miscarriage? Why I Don’t Talk About My Miscarriage 14 Onesies that Celebrate IVF Babies IVF Pregnancy Announcements Fertility Podcasts Tags Trying Pregnancy Miscarriage Health Difficulty Fetal Development Trending Articles Pregnancy IVF Pregnancy Announcements Egg Retrieval From Eggs to Blastocysts: Understanding IVF Attrition Cervical Mucus Identifying Stages of Cervical Mucus Artificial Insemination (AI) Implantation Calendar: What is Happening During the Two Week Wait?