Principles of ultrasonography
- Ultrasound uses sound waves delivered at high frequency (3.5–5 MHz for transabdominal and 5–7.5 MHz for transvaginal transducers). The higher the frequency the better the resolution, but the less the tissue penetration.
- Interpretation of images requires operator experience.
Indications (Figure 40.1)
Routine use of obstetric ultrasound can improve detection of fetal anomalies, accurately determine gestational age, and facilitate early diagnosis of multiple pregnancies. However, it is expensive and has not consistently been shown to improve perinatal outcome.
Complications
There are no confirmed adverse effects of ultrasound on the fetus. The major complication is false-positive and false-negative diagnoses.
Guidelines for obstetric ultrasound
First trimester sonography (Figure 40.2)
- Evaluate the uterus for the presence of a gestational sac. An intrauterine gestational sac should be seen at a serum βhCG (β-human chorionic gonadotropin) level of ≥1,000–1,200 mIU/mL by transvaginal scan and ≥6,000 mIU/mL by transabdominal ultrasound. If no intrauterine pregnancy is seen, the possibility of an ectopic pregnancy should be considered.
- If a gestational sac is identified, it should be examined for a yolk sac (usually evident at a βhCG of 7,000 mIU/mL) and embryo (at 11,000 mIU/mL).
- Gestational age should be documented. Crown–rump length (CRL) in the early first trimester is an accurate determinant of gestational age to within 3–5 days (compared with an error of ±2 weeks by second trimester measurements and ±3 weeks by third trimester ultrasound).
In the late first trimester, measurement of the biparietal diameter (BPD) can be used to estimate gestational age.
- Fetal cardiac activity
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