First-Trimester Obstetric Ultrasound

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SAMPLE EXCERPT
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PRE-PROCEDURE
INDICATIONS
  • Confirmation of pregnancy
  • Estimation of gestational age
  • Evaluation of fetal growth
  • Vaginal bleeding of undetermined origin
  • Suspected multiple gestation
  • Pelvic mass/pain
  • Suspected hydatidiform mole
  • Adjunct to cervical cerclage placement
  • Suspected ectopic pregnancy
  • Adjunct to special procedure
  • In vitro fertilization
  • Embryo transfer
  • Suspected fetal death
  • Suspected uterine abnormality
  • Intrauterine contraceptive device localization
  • Ovarian follicle development surveillance for infertility
  • Follow-up observation of identified anomaly*
  • History of previous infant with congenital anomaly*
  • Serial evaluation of fetal growth in multiple gestation
  • To measure nuchal translucency (NT) as part of a screening program for fetal aneuploidy*

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

CONTRAINDICATIONS
  • Maternal refusal
EQUIPMENT
  • 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
ANATOMY
  • Fetal size
    • Assessments are possible as early as 6 weeks gestation, when the fetus begins to uncurl as it develops.
    • Rudimentary limbs and internal organs are visible as early as 6 weeks, but it is difficult to measure the fetus before 9 weeks because it is very small and the boundaries are unclear.

Appearance of the Gestational Sac

  • The gestational sac (GS) consists of a hypoechogenic area and an echogenic rim or ring, which corresponds to the trophoblast. The GS of a normal pregnancy also may be characterized by a double-echogenic ring At the implantation site, the hyperechoic rim is thicker.
  • Major criteria for a normal-appearing gestational sac include the following:
    • A sac of 25 mm or greater in diameter must reveal an embryo within it.
    • The sac must be round.
  • Minor criteria for a normal-appearing gestational sac include the following:
    • The gestational sac is located in the fundus of the uterus.
    • A thick, echogenic decidual ring surrounds the gestational sac.
    • Evidence of the double-ring sign is found.
  • A gestational sac of abnormal size or appearance correlates highly with an abnormal outcome.
  • When a gestational sac with a mean diameter greater than 25 mm lacks an embryo, or when the gestational sac is grossly distorted, abnormal pregnancy is almost certain.
  • Once embryonic cardiac motion is seen on ultrasound, the likelihood of spontaneous abortion is very low.

PROCEDURE
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

POST-PROCEDURE
CARE
  • Advise the patient about whether follow-up scans or other management will be needed.
COMPLICATIONS
  • 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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