Part 2 book “Ultrasound in obstetrics and gynecology - A practical approach” has contents: Placental abnormalities, amniotic fluid assessment, ultrasound of the non- pregnant uterus, ultrasound evaluation of the adnexae, ectopic pregnancy, writing the ultrasound report, and other contents. | PLACENTAL ABNORMALITIES 8 INTRODUCTION The placenta develops from the trophoblast cell layer of the blastocyst embryo at about 6 days from fertilization. With attachment of the blastocyst to the endometrial cavity, the trophoblastic cells differentiate into an inner layer; the cytotrophoblasts and an outer layer; the syncytiotrophoblasts. The syncytiotrophoblasts develop lacunae forming early intervillous spaces. The placenta forms at the site of the chorion frondosum (the fetal portion of chorion) and the decidua basalis and is first recognized sonographically as a thickened echogenic region by about 9-10 weeks of gestation (Figure ). Maternal blood flow is established within the placenta by 12 weeks of gestation (1). The placenta at term is about 20 cm in diameter with a volume of 400 to 600 ml (2). In general, measurement of the placenta in not obtained currently unless in rare pathologic conditions and thus assessment of the biometric dimensions of the placenta are infrequently performed on prenatal sonography today. The normal thickness of the placenta is correlated to gestational age with approximately 1 mm per weeks of gestation (3). Figure : Ultrasound of an intrauterine pregnancy at 9 weeks showing the echogenic placenta (labeled). Note the decidua basalis (labeled) as a hypoechoic region behind the placenta. The embryo is also shown (labeled). Chapter 8: Placental Abnormalities 153 Placental localization by ultrasound is one of the six components of the standardized approach to the basic obstetric ultrasound examination and the technical detail of this examination is described in chapter 10. In this chapter, we will focus on the ultrasound diagnosis of placental abnormalities. PLACENTA PREVIA The term placenta previa describes a placenta that covers part or all of the internal cervical os. In normal pregnancy, the placenta implants in the upper uterine segment. In the case of placenta previa, the placenta is partially or totally implanted in
