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Female Pelvis, Pregnant: Second and Third Trimesters
Anatomy
This chapter is different from other chapters, in that there may be a variety of causes and different anatomic regions may be the etiology of pain in later pregnancy. As for anatomic considerations, the main focus of this chapter will be on obstetrical etiologies of pain and/or bleeding. Much of the focus will be on the placenta, cervix, umbilical cord, and amniotic fluid. During the second and third trimesters of pregnancy, the fetus continues to grow with concomitant changes in the placenta that connects the fetus to the uterus. The placenta develops from the chorion frondosum that has developed from the blastocyst of the embryo. It is attached to the uterus via the decidua basalis (from the maternal placenta). The placenta is the connection between the maternal circulation and the fetal circulation and allows for exchange of oxygen and nutrients between mother and fetus. The location of the placenta and its relationship to the cervix are important and will be discussed in more detail. The cervical is closed during pregnancy and effaces and shortens before birth. Premature opening of the cervix, placental abnormalities, and the relationship of the placenta to the cervix will be discussed in more detail in this chapter.
The umbilical cord is composed of one vein flowing toward the fetus and two arteries flowing from the fetus to the placenta. Normally, the umbilical cord should insert in the more central portion of the placenta. If the cord inserts on the placenta edge or is the presenting part during delivery, this can have devastating consequences on the pregnancy.
Amniotic fluid volume is dependent on the balance between the production and removal of amniotic fluid. In the second and third trimesters of pregnancy, the major source of fluid is production of urine by the fetal kidneys. In late pregnancy, amniotic fluid is reabsorbed into the gastrointestinal tract after fetal swallowing. Amniotic fluid is important for the well-being of the fetus.
Ultrasound Techniques/Findings
Placenta
The placenta appears uniform in echotexture and thickness between the eighth and twentieth weeks of pregnancy. At that time the placenta measures up to 3 cm in thickness. Later in pregnancy the placenta usually measures less than 4 cm in thickness. However, there may be myometrial contractions in which the placenta may appear artificially thickened. Later in pregnancy hypoechoic regions may develop within the placenta, as “placental lakes.” There may be other hypoechoic regions within the placenta, most of which are benign in etiology.
The placenta is usually attached in the mid-to-fundal portion of the uterus. Identification of the location of the placenta and its relationship to the cervix is of utmost importance. Overdistention of the urinary bladder and/or focal uterine contractions may give the false impression of a placenta previa by compressing the lower portion of the uterus.
The placenta is separated from the myometrium by a subplacental venous complex called the decidua basalis . This is a few millimeters in thickness and has different appearances depending on the location of the placenta. The decidua basalis is important to visualize because, with placenta accreta, placental tissue invades through this region into the myometrium.
Umbilical Cord
The placenta can be scanned and color flow may be utilized to identify the insertion of the cord into the placenta. Routine visualization of the location of the cord into the placenta is essentially the diagnosis of marginal and velamentous insertion of the umbilical cord. Color flow imaging is helpful in this identification and following the very circuitous course of the umbilical cord. Also important in the third trimester of pregnancy is to examine the cervix with color to make sure the umbilical cord does not overlay the cervical os.
Cervix
The appearance of the cervix is important when evaluating a patient with pain or bleeding in the second or third trimester of the pregnancy. Abdominal ultrasound examination of the cervix often may be difficult because of myometrial contractions or fetal parts overlying the lower portion of the uterus/cervix. In these cases translabial or transvaginal techniques may be helpful to image the cervix when the transabdominal approach is unsuccessful. Translabial scanning is performed with the patient’s bladder empty. A sterile glove or cover is placed over the probe using this approach. The translabial approach is useful to exclude placenta previa. Endovaginal scanning with the bladder partially full may be helpful when evaluating for such abnormalities as placenta previa or placenta accreta. These entities will be discussed in more detail later in the chapter. Cervical incompetence, shortening of the cervix, mild dilatation of the endocervical canal, or bulging of the amniotic membranes may be diagnosed with the transabdominal technique, but in many cases translabial and/or endovaginal scanning may be needed.
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