Third-Trimester Obstetric Ultrasound

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  • Estimation of gestational age
  • Evaluation of fetal growth
  • Vaginal bleeding of undetermined origin
  • Determination of fetal presentation/presenting part
  • Suspected multiple gestation
  • Adjunct to amniocentesis
  • Significant uterine size/dates discrepancy
  • Pelvic mass/pain
  • Suspected hydatidiform mole
  • Adjunct to cervical cerclage placement
  • Suspected fetal death
  • Suspected uterine abnormality
  • Biophysical profile (BPP)
  • Observation of intrapartum events
  • Suspected polyhydramnios or oligohydramnios
  • Suspected placental abruption
  • Adjunct to external cephalic version
  • Estimation of fetal weight and/or presentation and/or cervical dilation in premature rupture of membranes and/or preterm labor
  • Follow-up observation of identified anomaly*
  • Follow-up evaluation of placental location for identified placenta previa
  • History of previous infant with congenital anomaly*
  • Serial evaluation of fetal growth in multiple gestation
  • Evaluation of fetal condition in late registrants for prenatal care
  • Evaluation of fetal well-being by Doppler flow velocities in high-risk pregnancies*

*Usually, a targeted examination performed by individuals experienced in this area.

  • Maternal refusal
  • Real-time ultrasound machine with a 3-MHz or higher transducer for transabdominal scans, or a 5-MHz or higher transducer for transvaginal scans
  • Ultrasonic gel
  • Towels to remove gel when study completed
  • Sheaths or probe covers for transvaginal scanning
  • Appropriate forms for documentation

Fetal Presentation

  • As the time for delivery nears or during labor, the fetus usually shifts to a vertical lie.

Placental Anatomy

  • At term, the normal placenta is usually 15 to 20 cm in diameter and weighs about 500 g.
  • The placenta is divided into numerous lobes.
  • By the second month of development, only a single umbilical vein is present in the umbilical cord.
  • A placenta that is located over the maternal cervical os is a placenta previa.

Amniotic Fluid

  • Fetal lung tissue secretes some fluid, but fetal urine is the main source of fluid.
  • Direct measurement of amniotic fluid volume (AFV) is prone to inaccuracies, and the AFI offers a more reproducible measure.

Sample excerpt does not include step-by-step text instructions for performing this procedure
The full content of this section includes:
  • Step-by-step text instructions for performing the procedure
  • Clinical pearls providing practical clinical tips from medical experts
  • Patient safety guidelines consistent with Joint Commission and OHSA standards
  • Links to medical evidence and related procedures

  • Advise the patient if follow-up scans or other management will be needed.
  • Although theoretical risks for ultrasound damage to human fetuses are known, no proven harm to any human fetus or mother has been documented.
  • Failure to diagnose an anomaly or condition.
  • Inaccurate estimate of gestational age or weight.
  • Inappropriate reassurance of a perfect infant.
  • Inaccurate determination of the sex of the infant.
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