Updated: 12/30/2021

Fetal Parameters

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  • Ultrasound
    • Ultrasound most reliable method for assessing fetal growth
      • in the first trimester of pregnancy, measurement of the crown-rump length on ultrasound is the most precise method of dating
      • earlier ultrasounds are almost always more accurate than late ultrasounds
        • never change the estimated gestational age based on an ultrasound obtained late in the pregnancy
        • ultrasound is the next best step in management in
          • patient's with abnormal AFP, β-hCG, inhibin, unconjugated estriol levels
          • cases where you are unsure of fetal presentation
          • cases of painless vaginal bleeding and fetal bradycardia (vasa previa)
          • any case where you suspect placenta previa (painless vaginal bleeding, no fetal bradycardia)
    • Fetal stress test
      • used to assess fetal well-being
      • most common cause of abnormal fetal stress test is fetal sleep cycle
  • Fundal Height
    • Least invasive
    • If 2 cm deviation from normal during weeks 18-36, repeat measurement and/or ultrasound
      • Fundal Height
      • Timeline
      • Location of Fundus
      • 12 weeks
      • Pubic symphysis
      • 16 weeks
      • Midway from symphysis to umbilicus
      • 20 weeks
      • Umbilicus
      • 20-36 weeks
      • Height from pubic symphysis in cm correlates with weeks of gestation
  • Early Pregnancy Fetal Parameters
    • Useful in early pregnancy
      • gestational sac
      • crown-rump length
  • Late Pregnancy Fetal Parameters
    • Four measurement requirement due to wide deviation in normal for each
    • One of the most effective ways of evaluating gestational age
    • Includes
      • cerebellar diameter (HC)
      • biparietal diameter of skull (BPD)
      • abdominal circumference (AC)
      • femur length (FL)
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(M2.OB.17.4867) A 25-year-old G1P0000 presents to her obstetrician’s office for her first prenatal visit. She had a positive pregnancy test 6 weeks ago, and her last period was about two months ago, though at baseline her periods are irregular. Aside from some slight nausea in the mornings, she feels well. Which of the following measurements would provide the most accurate dating of this patient’s pregnancy?

QID: 109650

Biparietal diameter

7%

(1/15)

Femur length

13%

(2/15)

Abdominal circumference

0%

(0/15)

Serum beta-hCG

27%

(4/15)

Crown-rump length

47%

(7/15)

M 6 E

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