The placenta is a highly specialized organ that supports growth and development of the fetus and serves as the interface between the maternal and fetal circulations. The placenta functions as the pregnancy organ that delivers nutrients, exchanges respiratory gas, and eliminates toxic waste. The placenta is also an important endocrine organ producing hormones to support and sustain pregnancy and plays a critical role in prevention of pregnancy rejection. Impairment in placental development and/or function has a profound impact on pregnancy outcome. Accumulating data suggest that the placenta plays a critical role in the future health of the fetus such as the risk for adult-onset cardiovascular disease among other diseases.¹, ², ³, ⁴ This chapter presents the embryologic development and normal sonographic appearance of the placenta and umbilical cord and discusses common placental and cord abnormalities that can be detected in the first trimester of pregnancy.

The blastocyst reaches the endometrial cavity at 4 to 5 days postfertilization. The outer surface of the blastocyst differentiates into trophoblastic cells and produces an overlying syncytial layer that adheres to the endometrium. Implantation of the blastocyst then commences as the syncytiotrophoblast cells penetrate the decidualized endometrium. Endometrial gland secretions provide nourishment to the embryo at this early stage. Spaces are then developed within the syncytiotrophoblast and form anastomosis with maternal vascular sinusoids, thus establishing the first (lacunar) uteroplacental circulation (Fig. 15.1). The placental circulation then develops with finger-like projections into the maternal blood spaces. These projections extend from the chorion and form the primary villi with an inner layer of cytotrophoblast and an outer layer of syncytiotrophoblast. The primary villi become secondary villi with the invasion of the extraembryonic mesoderm and finally become tertiary villi as embryonic blood vessels develop within them. In the early stages of placental development, cytotrophoblasts invade the endothelium and smooth muscle of endometrial spiral arteries, releasing them from maternal influences. The fully formed human placenta is termed hemochorial because the maternal blood is separated from the fetal blood only by elements of the chorion. Growth in size and thickness of the placenta continues rapidly in the first trimester and into the second trimester of pregnancy. The term placenta has a fetal portion, the chorion frondosum, and a maternal portion, the decidua basalis, and covers 15% to 30% of the decidua of the endometrial cavity.⁵ The placenta at term is about 20 cm in diameter, has a volume of 400 to
600 mL, and weighs approximately one-sixth as much as the fetus.⁵,⁶

In early embryogenesis, two stalks are seen: the yolk sac, ventrally located and containing the vitelline duct and vessels, and the connecting stalk, caudally located and containing the allantois and the umbilical vessels (Fig. 15.2). With cephalocaudal flexion of the embryo, the connecting stalk fuses with the yolk sac stalk to form the umbilical cord. The umbilical cord in its early development is inserted in the lower ventral portions of the embryo, is short and thick, and contains the allantois, the vitelline duct and vessels, and the umbilical vessels. The amnion covers the umbilical cord and becomes continuous with the outer epithelial layer of the embryo. The umbilical cord elongates and thins out with the development of the anterior abdominal wall. The umbilical cord initially attaches centrally to the developing
placenta. As the placenta grows, it tends to expand preferentially in regions with sufficient myometrial perfusion and atrophy in areas with suboptimal blood supply. As a result, the cord insertion may become somewhat eccentric. This process is known as trophotropism. The umbilical cord consists of two umbilical arteries and one vein, which are surrounded by mucoid connective tissue—Wharton jelly. The cord at term is usually 1 to 2 cm in diameter and 30 to 90 cm in length.

The placenta is first recognized sonographically as a thickened echogenic region on the endometrium by about 9 to 10 weeks of gestation (Figs. 15.3 and 15.4A). By 12 to 13 weeks of gestation, the placenta is easily seen on ultrasound and appears slightly echogenic with uniformed homogeneous echotexture (Figs. 15.4B and 15.5).

During the first trimester ultrasound examination (see Chapter 5), the location of the placenta in the uterus should be reported. Identifying the location of the placenta on the first trimester ultrasound is not as easy as in the second trimester given the presence of uterine flexion and extension. Indeed, inaccuracies can be introduced especially when the placenta appears to be in the lower uterine segment. In order to improve accuracy of placental localization on the first trimester ultrasound, we recommend identifying the cervix and the anterior and posterior uterine walls before describing the placental location (Figs. 15.6 and 15.7).

The placental size, thickness, location within the endometrial cavity, and echogenicity can also be evaluated by ultrasound in the late first trimester of pregnancy.⁷ The normal thickness of the placenta is correlated to gestational age of the embryo/fetus and is approximately 1 mm per week of gestation.⁸ With advanced three-dimensional (3D) ultrasound techniques, placental volume can now be measured in the first trimester
(Fig. 15.8), and along with placental biometry has been shown to correlate with pregnancy complications.⁹,¹⁰ Indeed, we believe that first trimester measurement of placental length, width, and volume is more accurate than later on in pregnancy given that the whole placenta is seen in one ultrasound image in early gestation. The assessment of biometric dimensions of the placenta is infrequently performed on prenatal sonography today, unless in rare pathologic conditions or for research purposes. Abnormal placental findings on first trimester ultrasound, such as masses, multiple cystic spaces, or large subchorionic fluid collection, should be noted and followed up.

Maternal blood flow is established within the placenta by 12 weeks of gestation.¹¹ Blood flow in the maternal and fetal placental vasculature can be demonstrated in the first trimester on color and pulsed Doppler ultrasound (Fig. 15.9).

Quantitative assessment of placental vascularization may be useful for predicting pregnancy complications and adverse events.¹², ¹³, ¹⁴

The umbilical cord can be recognized by ultrasound as early as the seventh week of gestation and appears as a straight thick structure connecting the embryo to the developing placenta (Fig. 15.10). In the first trimester, the length of the umbilical cord is approximately the same as the crown-rump length.¹⁵ The umbilical cord elongates and thins with advancing gestation, and by the 13th week, the cord has the same sonographic appearance as in the second trimester of pregnancy (Fig. 15.11). Umbilical arteries can be seen in the first trimester as branches of the internal iliac arteries, running alongside the fetal bladder in a cross-section view of the fetal pelvis using color or power Doppler (Fig. 15.12A), and the number of umbilical arteries can thus be reliably determined.¹⁶ Midline sagittal plane of the fetus demonstrates the insertion of the umbilical cord into the abdomen, and this plane is also important in the evaluation of the integrity of the abdominal wall in the first trimester (Fig. 15.12B) (see Chapters 5 and 12). 3D ultrasound in surface mode can also clearly demonstrate the umbilical cord (Fig. 15.13).