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.
Transvaginal cervical length of a 24-week uterus. The hyperechoic line identifies the glandular tissue of the cervix and correct positioning to include the entire cervix. Measurement is from the internal os traced out to the external os. The fetal head is visible in this shot but not necessary for the measurement. (Reproduced with permission from Danielle Cooper, MD.)
Ultrasound examination of a 21-week estimated gestational age (EGA) fetus showing measurement of the baby's biparietal diameter (BPD) and head circumference (HC). Note the presence of the falx cerebri and the thalamus (TH) to ensure the measurement is at the right anatomic level. (Reproduced with permission from E.J. Mayeaux, Jr., MD.)
Ultrasound examination of a 21-week estimated gestational age (EGA) fetus showing measurement of the baby's abdominal circumference (AC). Note how the stomach (St), spine (SP), and portal vein (PV) are all visible to ensure that the measurement is at the right anatomic level. (Reproduced with permission from E.J. Mayeaux, Jr., MD.)
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.
INDICATIONS FOR ULTRASOUND
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....