In a “normal” menstrual cycle of 28 days, days 1-14 are the follicular phase during which the ovary responds to follicle stimulating hormone (FSH) secreted by the pituitary gland [4]. FSH promotes development of a group of follicles that enlarge and produce and release estrogen.


Rising estrogen levels suppress secretion of FSH, resulting in atresia of all but one of the stimulated follicles. This dominant follicle (graafian follicle) continues to mature and enlarge and will be the source of ovulation. Ovulation occurs around day 14 (range, day 10-18) when the dominant follicle ruptures, releasing the ovum and a small amount of fluid into the peritoneal cavity. The corpus luteum develops at the site of the dominant follicle, forming a highly vascular mass that often becomes cystic as it fills with blood and lymph. The corpus luteum produces progesterone that maintains the secretory phase endometrium for successful implantation during the second half of the menstrual cycle. If fertilization of the ovum fails to occur, the corpus luteum will degenerate within 14 days. If fertilization occurs, human chorionic gonadotropin (hCG) from the fertilized ovum stimulates the corpus luteum to continue hormone secretion to support the developing pregnancy. The corpus luteum continues to function through the tenth week of gestation when the placenta replaces its hormone production. The corpus luteum may persist as a visible structure throughout pregnancy and must be recognized and differentiated from pathologic structures.

A zygote is formed when a sperm fertilizes the ovum, usually in the ampullary portion of the fallopian tube. Cell division occurs as the zygote migrates to the endometrial cavity. A
cystic structure called the blastocyst is formed with two cell layers present. The outer cell layer is the trophoblast that forms the chorion and the fetal components of the placenta. The inner cell layer forms the embryo, umbilical cord, amnion and secondary yolk sac. The blastocyst implants 7-10 days after fertilization (around menstrual day 23) by burrowing into the endometrium. The thickened, secretory-phase endometrium, characteristic of the second half of the menstrual cycle, begins to secrete glycogen rich in mucin to support the pregnancy. The thickening and functional change of the endometrium in response to pregnancy is called decidual reaction, and the endometrium is now called decidua. The blastocyst continues to develop forming a recognizable GS completely covered by decidua.

The normal anatomy of the GS must be recognized to correctly interpret early pregnancy US examinations (Fig. 6.12). The maternal decidua consists of three layers (Fig. 6.13). The decidua capsularis covers the surface of the expanding GS as it protrudes into the uterine cavity. The decidua basalis combines with chorion frondosum to form the placenta at the site of implantation. The decidua vera (also called decidua parietalis) lines the remainder of the uterine cavity away from the placenta. Because the GS is implanted within and is covered
by decidua, the uterine cavity remains open to the cervix. Bleeding associated with the pregnancy occurs into the uterine cavity and exits the uterus via the cervix. The outer membrane of the GS is the chorion. The inner membrane is the amnion (Fig. 6.10), which defines the amniotic cavity containing the developing embryo. The yolk sac is in the chorionic cavity, between amnion and chorion (Fig. 6.10). Low-level echoes are commonly seen within the fluid of the chorionic cavity. As the embryo grows, the amnion expands and becomes adherent to the chorion, obliterating the chorionic cavity at 15-16 weeks. Occasionally the amnion and chorion remain unfused throughout pregnancy. US features of the normal GS are listed below:

Radioimmunoassay (RIA) for the serum beta subunit of human chorionic gonadotropin (β-hCG) provides a specific and sensitive laboratory test for the presence of a fertilized ovum. If the RIA is negative, the patient is not pregnant. If RIA is positive, serial quantitative measurements are useful in diagnosis of normal and abnormal pregnancy. The problem comes in interpretation of results. Numerous papers have been written touting the value of “discriminatory levels” of β-hCG. For example, one study states that if serum β-hCG exceeds 1000 mIU/mL, then an intrauterine GS should always be identified in a normal pregnancy on TV US [18]. To apply this criterion, the first pitfall is to correlate the reporting standard used in the published study with the reporting standard used by the laboratory where you practice. Since the 1960s, a number of reporting standards have been utilized, including the First International Standard, the Second International Standard, the Third International Standard, and the International Reference Preparation. Reported values differ significantly depending on which standard is used. The second pitfall is that different laboratories using the same standard will report different results depending on which laboratory test kit they use. The range of normal values for a given week of pregnancy varies widely from one laboratory to another. These pitfalls make the use of reported discriminatory levels hazardous in any given practice unless specific correlation has been individually studied for a given laboratory and patient population. Further caution is warranted because even if these studies are correlated at each practice location, hospitals commonly change laboratories and test kits for cost considerations without informing practicing physicians.