Fertilization occurs at approximately 14 days after the beginning of the last menstrual period (LMP). Within 3–4 days, the zygote (fertilized egg) has traveled along the fallopian tube and reached the uterus, and, via cell division, has grown to a sphere of 12–15 cells. Over the same time period, the endometrium becomes thicker and richer in blood vessels, ready to support the developing pregnancy. This change in the endometrium occurs because of stimulation from the hormone human chorionic gonadotropin (hCG), which is produced by the corpus luteum, the remnant of the ovarian follicle that released the ovum before fertilization. The altered endometrium is called the “decidua.” By 5–6 days after fertilization, the collection of cells (now called a blastocyst) implants into the decidua. At about 2 weeks after fertilization, close to the expected time of the next menses, a blood or urine pregnancy test (which checks for the presence of hCG in these fluids) first becomes positive.

By convention, gestational age is measured as the time since the first day of the LMP, and hence is approximately 2 weeks more than the time since fertilization. The gestational sac that is implanted in the decidua is approximately 2 mm in mean diameter 3 weeks after fertilization, or at 5 weeks gestational age. The sac grows to 10 mm at 6 weeks gestation. Its outer rim is formed by the chorion, made up largely of trophoblastic tissue. The amnion, a second, thinner membrane, is initially in close contact with the developing embryo. The yolk sac, which provides nutrition to the embryo, lies immediately adjacent to the amnion and embryo, in the fluid space between the chorion and the amnion. Before 6 weeks gestation, the embryo is microscopic in size (<1 mm).

The gestational sac can first be visualized at approximately 5 weeks gestation when scanning transvaginally. It is often identifiable in a normal singleton pregnancy by the time the maternal serum hCG concentration reaches a level of 1,000 mIU/mL (first or third International Preparation), but the hCG concentration at which the gestational sac is first identifiable varies considerably from pregnancy to pregnancy. Thus, one cannot reliably rule out a normal intrauterine pregnancy (IUP) simply because the hCG concentration is above a certain value (“discriminatory level”) and no gestational sac is seen within the uterus on ultrasound.

At 5 weeks, the gestational sac appears as a fluid collection in the midportion of the uterus, within the echogenic decidua. The appearance of the gestational sac is quite
variable. In some cases, the gestational sac is surrounded in part by two echogenic rings (Figure 1.1.1), an appearance termed the “double sac sign.” In other cases, there is a thin bright line running down the middle of the decidua (representing the collapsed uterine cavity) and the gestational sac lies in the decidua on one side of this line (Figure 1.1.2), an appearance termed the “intradecidual sign.” In still other cases, the fluid collection has a nonspecific appearance, with neither of these signs (Figure 1.1.3). Since the double sac and intradecidual signs may be present or absent in an IUP, their presence or absence has little if any clinical value, and any round or oval fluid collection in the miduterus in a woman with a positive serum hCG should be interpreted as a probable gestational sac.

At approximately 5.5 weeks, the yolk sac, a circular structure that is normally less than 6 mm in diameter, is first identifiable within the gestational sac by transvaginal sonography (Figure 1.1.4). When scanning transabdominally, the gestational sac and the yolk sac are visualized approximately one-half week later than they are transvaginally.

The corpus luteum can usually be seen on ultrasound in one of the ovaries. It can have a number of sonographic appearances, including a simple cyst (Figure 1.1.5A), a thick-walled (Figure 1.1.5B) or complex cyst, or a solid hypoechoic structure. It typically measures 2–3 cm in diameter.

Between 6 and 10 weeks of gestation, the embryo undergoes rapid growth and development. It increases approximately 15-fold in length, from 2 mm at 6 weeks of gestation to 30 mm at 10 weeks. The internal organs differentiate, with organogenesis largely complete by 10 weeks. In particular, by 10 weeks, the cardiac chambers and valves are well formed, the gastrointestinal and genitourinary systems (which were joined at the urogenital sinus early in embryologic development) are distinct, and the kidneys have begun their ascent from the pelvis.

The external appearance of the embryo also undergoes major changes during this stage. By 10 weeks, facial features are recognizable, and limbs, including fingers and toes, are formed.

During the 6–10-week time period, the chorionic villi at the implantation site proliferate, whereas those opposite the implantation site regress. This results in two portions of the chorion: the thick chorion frondosum, where the villi have proliferated, and the smooth membranous chorion laeve, where the villi have degenerated. The chorion frondosum interdigitates with the maternal decidua to form the placenta. The thin smooth portion forms the chorionic membrane (often referred to simply as the chorion).