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A 23-year-old pregnant gravida 2, para 1 woman is being seen for ultrasound because of her uncertain dates. Her best recollection of her last period gave her an estimated gestational age (EGA) of 19 weeks. Her vital signs are normal and her fundal height is 20 cm. Figures 80-1, 80-2, 80-3 are still images taken from her ultrasound examination demonstrating an EGA of 21 weeks and 6 days by measurement of the baby's biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL). All four measurements allow the computer to calculate an estimated fetal weight of 431 g. The pregnancy proceeded without complications, and the patient delivered a healthy boy at 403/7 weeks based on the ultrasound-calculated estimated date of delivery. No interventions were needed for postdates because of the ultrasound calculations earlier in the pregnancy.
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The ideal time to perform an anatomic survey is between 18 and 20 weeks' gestation. The fetus has developed all of its organ systems and is large enough that these can be visualized in detail. Fetal biometry can be obtained to calculate a gestational age and has an accepted error margin of ±7–10 days. If resources are limited or patient costs are an issue, the second trimester ultrasound provides the most bang for the buck.
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INDICATIONS FOR ULTRASOUND
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Second and third trimester ultrasound examination can be used to determine fetal number and presentation, and for documentation of fetal cardiac activity, placental location, and amniotic fluid volume.
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