Obstetrics 2: Ultrasound Use Portal
Diagnostic ultrasound is helpful in addressing a number of important clinical questions and in assisting with common procedures that arise during labor and delivery. Regardless of diagnostic ultrasound availability in an imaging department, its ready access in labor and delivery is appropriate in any setting, large or small. Applications in labor and delivery often arise suddenly, at odd hours of the day and night. Ultrasound diagnosis may complement the clinical information available to the managing practitioner. The Emergency Department ultrasound machine is a suitable OB scanner—use the deep transducer.
NIH Indications for Obstetric Ultrasound
- Gestational age assessment in cases of uncertain dates, pregnancy termination, need for labor induction, or repeat
- Cesarean section before onset of labor
- Suspected ectopic pregnancy or pregnancy inpatients with known tubal disease or who have had surgery or previous ectopic pregnancies
- Size or date discrepancy
- Suspected uterine abnormality
- Suspected multiple gestation
- Suspected hydatidiform mole
- Suspected fetal death
- Maternal pelvic masses
- Vaginal bleeding; rule out placenta previa or placental abruption
- Repeat evaluation of placental location
- Fetal weight estimation in cases of premature labor, premature rupture of membranes, or suspected macrosomia
- Follow-up investigation of abnormal maternal serum alpha-fetoprotein
- Late start of prenatal care
- Determine fetal presenting part
- History of a previous infant with a congenital abnormality
- Intrauterine contraceptive device in the presence of a pregnancy
- Suspected polyhydraminion or oligohydramnios
- Guidance of amniocentesis or chorionic villus sampling
Basic Applications of Labor and Delivery Ultrasound
- Diagnosis of Fetal Position
By making a series of transverse and longitudinal sweeps of the maternal abdomen, the presenting part of the fetus (head vs. breech) is almost always clear. If the fetal lie is transverse and the patient is in labor, knowing if the spine is up or down in relation to the lower uterine segment is helpful. The relationship is significant because the risk of cord prolapse is increased if the spine is up. Additionally, the position of the spine of the fetus may affect the choice of uterine incision at the time of Cesarean section. The back down position often requires a vertical or classical uterine incision during Cesarean section. - Fetal Number
Search all quadrants of the uterus thoroughly to detect multiple gestation. You should be able to detect two separate heads, spines, and fetal heart beats before making a multiple gestation diagnosis. - Presence of Fetal Life
Fetal life may be easily diagnosed by observing fetal cardiac motion with real time scanning. Regardless of their experience, most clinicians prefer to share the burden of diagnosis of fetal death by having another examiner confirm the absence of fetal cardiac activity over several minutes of observation. Typically, a formal ultrasound with a recording is best.
Characteristics of Fetal Demise
- No cardiac motion
- Hydropic changes
- Abnormal lie
- Overlapping skull bones
- Oligohydramnios
- Placental Location
The diagnosis of placental location and appearance is important when placenta previa or placental abruption are suspected and prior to Cesarean section if a low anterior implantation is suspected. The placenta is typically more echogenic than the myometrium, but myometrial contractions can alter the apparent location, appearance, and thickness of the placenta. Furthermore, crowding and oligohydramnios can make detecting the placenta difficult. Posterior locations are obscured by acoustic shadows of the fetus; succenturiate lobes may be difficult to find. Placenta previa may be difficult to diagnose sonographically, depending on the location of the presenting part, the quantity of amniotic fluid, and the amount of urine in the maternal bladder. If the bladder is over-distended, the lower uterine segment may be compressed, creating a false impression of placenta previa.
Ultrasound evaluation of the lower uterine segment for possible placenta previa is best performed with the bladder partially filled. Try to time the evaluation during an interval without uterine contractions. Careful transvaginal scanning with a 5.0 MHz or a 7.5 MHz transducer may be done if the patient is not actively bleeding and the cervical os is closed on speculum examination. Alternately, scanning with a condom- or glove-covered 3 to 5 MHz transducer placed at the introitus or on the perineum can be helpful. Transperineal or transvaginal scanning approaches may provide additional information about the relationship of the placenta to the cervical os and about the length of the cervix.
- Amniotic Fluid Aassessment
An assessment of amniotic fluid provides information about the overall well being of the pregnancy. Specifically, oligohydramnios may be supportive information for rupture of membranes if the diagnosis is in question. To measure the amniotic fluid index, the ultrasound probe must be held absolutely perpendicular to the abdomen; an amniotic fluid pocket is measured in each of the 4 quadrants of the abdomen. Only fluid pockets that do not contain umbilical cord or fetal small parts may be measured. The sum of the fluid measurements in each of the 4 quadrants is the amniotic fluid index. An abnormally low index is < 5 cm while 5 to 8 cm is borderline. Over 20 cm indicates polyhydramnios.
Extended Applications of Labor and Delivery Ultrasound
Examination for Placental Abruption
Placental abruption may have variable appearances, depending on
gestational age. The amount of bleeding is often underestimated.
Clotted blood may exhibit echogenicity similar to that of placental
tissue. Fresh hemorrhage may appear as a sonolucent area between the
uterine wall and the placenta or in the substance of the placenta.
Non-pathologic venous lakes and marginal sinuses may have a similar
appearance. Placental abruption may also take the form of an abnormal
thickening or rounding of the placental edge, presumably from a
marginal separation. The failure to diagnose placental abruption
sonographically in a suspected clinical setting does not
exclude the diagnosis. Therefore, the physician should not change
management based solely on a negative ultrasound scan.
Conclusion
The ability to rapidly assess fetal
presentation, fetal number, viability, placental location, and possible
placental abruption through basic applications of diagnostic ultrasound
in labor and delivery can be learned rapidly and have a significant
bearing on clinical management.