Ultrasound studies of the obstetric patient are valuable for (1) confirming pregnancy; (2) facilitating amniocentesis by locating a suitable pool of amniotic fluid; (3) determining fetal age; (4) confirming multiple fetuses; (5) ascertaining whether fetal growth is normal, through sequential studies; (6) determining fetal viability; (7) localizing placenta; (8) confirming masses associated with pregnancy; (9) identifying postmature pregnancy (increased amount of amniotic fluid and degree of placental calcification); (10) serving as a guidance method for chorionic villus sampling (CVS), embryo transfer, intrauterine device (IUD) extraction, and percutaneous umbilical vein sampling; and (11) determining fetal nuchal translucency. A pregnancy can be dated with considerable accuracy if an ultrasound is done at 20 weeks’ gestation and a follow-up scan is done at 32 weeks’ gestation. This validation is most important when early delivery is anticipated and prematurity is to be avoided. Conditions in which determination of pregnancy duration is useful include maternal diabetes, Rh immunization, and preterm labor (Chart 13.2).

The pregnant uterus is ideal for echographic evaluation because the amniotic fluid–filled uterus provides strong transmitting interfaces between the fluid, placenta, and fetus. Ultrasonography has become the method of choice for evaluating the fetus and placenta, eliminating the need for the potentially injurious x-ray studies that were used previously.

1. A transvaginal (endovaginal) approach is most commonly used during the first trimester of pregnancy. No patient preparation is required for this method.

2. A transabdominal approach in used during the second trimester of pregnancy. Exceptions are made when the scan is performed to locate the placenta before amniocentesis, for evaluation of an incompetent cervix, or during labor and delivery. With this approach, the patient will need to have a full bladder. The patient is asked to drink five to six glasses of fluid (water or juice) about 1–2 hours before the examination. If she is unable to do so, intravenous fluids may be administered. She is asked to refrain from voiding until the examination is complete. Tell the patient that she will have a strong urge to void during the examination. Discomfort caused by pressure applied over a full bladder may be experienced. If the bladder is not sufficiently filled, three to four 8-oz glasses of water should be ingested, with rescanning done 30–45 minutes later. A full bladder allows the examiner to assess the true position of the placenta, repositions the uterus, and acts as a sonic window to the pelvic organs.

3. Have the pregnant woman lie on her back with her abdomen exposed during the test. This may cause some shortness of breath and supine hypotensive syndrome, which can be relieved by elevating the upper body or turning the patient onto her side.

4. For the transvaginal (endovaginal) procedure, a slim transducer, properly covered and lubricated, is gently introduced into the vagina. Because the sound waves do not need to traverse abdominal tissue, exquisite image detail is produced.

5. For the transabdominal approach, a coupling agent (special transmission gel, lotion, or mineral oil) is liberally applied to the skin to prevent air from absorbing sound waves. The sonographer slowly moves the transducer over the entire abdomen to obtain an image of the uterine contents.

6. Tell the patient that the examining time is about 30–60 minutes.

7. See Chapter 1 guidelines for intratest care.

1. During the first trimester, the following information can be obtained:

   1. Number, size, and location of gestational sacs

   2. Presence or absence of fetal cardiac activity and body movement

   3. Presence or absence of uterine abnormalities (e.g., bicornuate uterus, fibroids) or adnexal masses (e.g., ovarian cyst, ectopic pregnancy)

   4. Pregnancy dating (e.g., biparietal diameter, crown–rump length)

2. During the second and third trimesters, ultrasound can be performed to obtain the following information:

   1. Fetal viability, number, position, gestational age, growth pattern, and structural abnormalities

   2. Amniotic fluid volume

   3. Placental location, maturity, and abnormalities

   4. Uterine fibroids and anomalies

   5. Adnexal masses

   6. Early diagnosis of fetal structural abnormalities

3. Fetal viability: Fetal heart activity can be demonstrated at 5 weeks’ gestation in most cases. This information is helpful in establishing dates and in the management of vaginal bleeding. Molar pregnancies (a nonviable fertilized egg implants into the uterus; the pregnancy will not come to term) and incomplete, complete, and missed miscarriages can be 
differentiated.

4. Gestational age: Indications for gestational age evaluation include uncertain dates for the last menstrual period, recent discontinuation of oral hormonal suppression of ovulation, bleeding episode during the first trimester, amenorrhea of at least 3 months’ duration, uterine size that does not agree with dates, previous cesarean birth, and other high-risk conditions.

5. Fetal growth: The conditions that serve as indicators for ultrasound assessment of fetal growth include poor maternal weight gain or pattern of weight gain, previous intrauterine growth retardation (IUGR), chronic infection, ingestion of drugs such as anticonvulsants or heroin, maternal diabetes, pregnancy-induced or other hypertension, multiple pregnancy, and other medical or surgical complications. Serial evaluation of biparietal diameter and limb length can help differentiate between wrong dates and IUGR. Doppler evaluation of the umbilical artery, uterine artery, and fetal aorta can also assist in the detection of IUGR. IUGR can be symmetric (the fetus is small in all measurements) or asymmetric (head and body growth vary). Symmetric IUGR may be caused by low genetic growth potential, intrauterine infection, maternal undernutrition, heavy smoking by the mother, or chromosomal anomaly. Asymmetric IUGR may reflect placental insufficiency secondary to hypertension, cardiovascular disease, or renal disease. Depending on the probable cause, the therapy varies.

6. Fetal anatomy: Depending on the gestational age, the following structures may be identified: intracranial anatomy, neck, spine, heart, stomach, small bowel, liver, kidneys, bladder, and extremities. Structural defects may be identified before delivery. The following are examples of structural defects that may be diagnosed by ultrasound: Hydrocephaly, anencephaly, and myelomeningocele are often associated with polyhydramnios (excessive accumulation of amniotic fluid; occurs in <1% of pregnancies). Potter syndrome (renal agenesis) is associated with oligohydramnios defects (dwarfism, achondroplasia, osteogenesis imperfecta) and diaphragmatic hernias. Other structural anomalies that can be diagnosed by ultrasound are pleural effusion (after 20 weeks), intestinal atresias or obstruction (early pregnancy to second trimester), hydronephrosis, and bladder outlet obstruction (second trimester to term with fetal surgery available). Two-dimensional (2D) studies of the heart, together with echocardiography, allow diagnosis of congenital cardiac lesions and prenatal treatment of cardiac arrhythmias.

7. Detection of fetal death: Inability to visualize the fetal heart beating, lack of fetal movement, and overlapping of skull bones (Spalding sign) are signs of death.

8. Placental position and function: The site of implantation (e.g., anterior, posterior, fundal, in lower segment) can be described, as can location of the placenta on the other side of midline. The pattern of uterine and placental growth and the fullness of the bladder influence the apparent location of the placenta. For example, when ultrasound scanning is done in the second trimester, the placenta seems to be overlying the os in 15%–20% of all pregnancies. At term, however, the evidence of placenta previa (placenta is partially in lower uterine segment) is only 0.5%. Therefore, the diagnosis of placenta previa can seldom be confirmed until the third trimester. Placenta abruptio (premature separation of placenta) can also be identified. A transverse scan through the umbilical cord confirms the number of vessels. Doppler of the cord detects flow abnormalities.

9. Fetal well-being: Ultrasound findings are a major component of the biophysical profiles. The following physiologic measurements can be accomplished with ultrasound: heart rate and regularity, fetal breathing movements, urine production (after serial measurements of bladder volume), fetal limb and head movements, and analysis of vascular wave forms from fetal circulation. Fetal breathing movements are decreased with maternal smoking and alcohol use and increased with hyperglycemia. Fetal limb and head movements serve as an index of neurologic development. Identification of amniotic fluid measuring at least 1 cm is associated with normal fetal status. The presence of one pocket measuring less than 1 cm or the absence of a pocket is abnormal; it is associated with increased risk of perinatal death.

10. Assessment of multiple pregnancy: Two or more gestational sacs, each containing an embryo, may be seen after 6 weeks. Of twin pregnancies diagnosed in the first trimester, only about 30% will deliver twins, owing to loss or absorption of one fetus. Of value is assessment of the relative fetal growth of twins when IUGR or twin-to-twin transfusion is suspected. One cannot unequivocally diagnose whether twins are monozygotes (identical; develop from one zygote) or heterozygotes (fraternal; two eggs are fertilized) with ultrasound alone unless fetuses of opposite sex are evident.

11. If the fetal position and amniotic fluid volumes are favorable, fetal sex can be determined by visualization of the genitalia. It must be cautioned, however, that sex determination is not the purpose of an obstetric ultrasound.

Pretest Patient Care

1. Provide a brief explanation of the procedure to be performed, emphasizing that it is not uncomfortable or painful and does not involve ionizing radiation that might be harmful to the mother or fetus.

2. Explain that the transducer will be lubricated before insertion into the vaginal and that there should only be minimal discomfort (transvaginal approach).

3. Explain that a liberal coating of coupling agent must be applied to the skin so that there is no air between the skin and the transducer and to allow for easy movement of the transducer over the skin. A sensation of warmth or wetness may be felt. The couplant (ultrasound gel) does not stain or discolor clothing, but the patient may prefer to don a gown (transabdominal 
approach).

4. Tell the patient that the sonographer may explain the images on the screen in basic terms. A photograph or video recording may be provided (per institutional policy).

5. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

1. Artifacts may be produced when the transducer is moved out of contact with the skin. This can be resolved by adding more coupling agent to the skin and repeating the scan.

2. Artifacts (reverberation) may be produced by echoes emanating from the same surface several times. This can be avoided by careful positioning of the transducer.

3. A posterior placental site may be difficult to identify because of the angulation of the reflecting surface or insufficient penetration of the sound beam owing to the patient’s size.

Normal

• Normal image of placental position, size, and structure

• Normal fetal position and size with evidence of fetal movement, cardiac activity, and breathing activity

• Adequate amniotic fluid volume

• Normal fetal intracranial, thoracic, and abdominopelvic anatomy; four limbs visualized