Early Pregnancy: Normal and Abnormal

Steven R. Goldstein, MD

Disclosures

Semin Reprod Med. 2008;26(3):277-283. 

In This Article

Gestational

Although the blastocyst begins to implant in the endometrium at 3 weeks menstrual age (1 week after conception), the first definitive ultrasound sign of pregnancy is the "gestational sac." This is a sonographic not an anatomic term. Before the appearance of the gestational sac, the endometrium is markedly echogenic and the arcuate vessels are somewhat prominent. This, however, is nondiagnostic and can often be seen in the normal late secretory phase. In ultrasound images, the gestational sac (Figure 4) appears as a thick echogenic rind surrounding a sonolucent center.

Figure 4.

Transvaginal ultrasound view of the uterus clearly showing the echogenic decidualized endometrial cavity. Located within it is an echogenic line (fiercely white circle) around a sonolucent center (black). The fiercely echogenic area is the result of primary trophoblastic villi invading into maternal decidua creating a chorionic decidual reaction.

Sonographically, the chorionic sac (gestational sac) is embedded in the depth of the thick endometrium (decidua) and appears on one side of the cavity line, not in the middle of it (Figure 5). This sonolucent center is actually the fluid-filled chorionic sac. This sac already contains the amnion, bilaminar embryonic disk, and yolk sac, but these structures are too small to be imaged even with the high magnification of our current scanners. The echogenic rind is the result of a trophoblastic decidual reaction. Early on, the entire chorionic sac is surrounded by chorionic villi (Figure 6). These villi are symmetrically located. Some villi will bud and branch into secondary and tertiary villi and become chorion frondosum (the forerunner of the placenta). Other primary villi will regress and become chorion laeve. The appearance of the gestational sac predates the fusion of the decidua parietalis to decidua capsularis.

Figure 5.

Highly magnified transvaginal ultrasound view showing the decidualized endometrium (labeled). The endometrial echo is a linear structure (labeled). Gestational sac is seen here to be burrowing into the posterior endometrium and is labeled "eccentric."

Figure 6.

Pathologic specimen of a pregnancy at 5 to 6 weeks LMP. The primary chorionic villi are seen protruding from the chorionic membrane. Some of these villi bud and branch into secondary and tertiary villi and become chorion frondosum. Others of the villi regress and become chorion leave.

The sac grows ∼1 mm/day in mean diameter during early pregnancy.[6] With further growth, first the yolk sac and later the embryo become visible sonographically inside the chorionic cavity. The yolk sac has a very bright echogenic rim around a sonolucent center. When it first appears at ∼5 weeks, it may only be 1 to 3 mm in diameter (Figure 7). Discriminatory size of the gestational sac for transvaginal visualization of the yolk sac is reported from 5 to 13 mm in mean sac diameter,[7] and transducer frequency contributes to the earliest size gestational sac (threshold level) as well as the largest size gestational sac in which a normal yolk sac should absolutely be seen (discriminatory level).

Figure 7.

Transvaginal ultrasound image showing highly magnified view of an intrauterine gestation at 5 to 6 weeks LMP. The gestation sac is clearly visualized. Within there is a 3-mm yolk sac barely visible.

The embryo has been present since 9 days after conception.[8] Sonographically, it is first recognized as a thickening along the yolk sac (Figure 8).

Figure 8.

Highly magnified transvaginal ultrasound view of an intrauterine pregnancy. The yolk sac is clearly visible with the thickening of the embryo seen along its lateral border.

Embryo is derived from the Greek word for "metamorphosis." The embryonic period begins at 3 weeks after conception (5 weeks last menstrual period [LMP]) when the cardiovascular system and the central nervous system begin to form. It is the period of organogenesis (thus, concerns about teratogens are appropriate) and morphogenesis-the development of shape.

Before 18 mm, a true "crown" and "rump" do not exist. Measurements of embryonic size (ES) are actually the greatest measurement along the long axis of the embryo. Initially, at the somite stage, the embryo is a linear structure 2 to 3 mm in length. The rostral neuropore closes and develops into the forebrain prominence and then the head. The caudal neuropore elongates into a tail. As it grows, the embryo is a C-shaped tadpole-like structure (Figure 9). The primitive heart has great prominence, which is why, early on, one can often detect the pulsations of cardiac activity before visualizing an embryo distinct from the adjacent yolk sac. With further development, the tail regresses, the head unfolds from its flexed position, and limb buds develop and are replaced by hands and feet. By 17 to 22 mm, we are measuring a recognizable crown-rump length (CRL) (Figure 10). Before 17 mm, the measurements of ES are the greatest measurement along the long axis of the embryo. By 10 weeks LMP, the further unfolding of the head, final regression of the tail, regression of the heart prominence, and development of the limbs and eyes allow this primitive tadpole-like form to now be recognizable as human in form-hence, fetus (from the Latin word for "offspring"). Thus, the fetal period begins at 70 days LMP (Figure 11).

Figure 9.

Transvaginal ultrasound image showing intrauterine pregnancy at 55 days LMP. This 13-mm embryo is a C-shaped tadpole-like structure.

Figure 10.

Transvaginal ultrasound image showing intrauterine gestation at 59 days LMP. Calipers clearly mark a crown to rump length (CRL) of 17 mm. Yolk sac is seen just cephalad to the crown.

Figure 11.

Transvaginal ultrasound image showing intrauterine gestation at 10 weeks LMP. This fetus (CRL = 35 mm) is now totally recognizable as a human offspring.

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