Ultrasound Pregnancy Dating

1. First Trimester (Transvaginal Ultrasound)
   1. Gestational Sac (5 weeks gestation, bHCG 1800)
      1. Round sac with echogenic ring located in fundus
      2. Age on mean diameter: (length + width + height)/3
   2. Yolk Sac (6 weeks gestation)
      1. Echogenic, round sac within Gestational Sac
      2. May appear more as '=' sign rather than circle
      3. Presence rules out pseudogestational sac of ectopic
   3. Embryo (7 weeks gestation, bHCG 20,000)
      1. Onset as dot at Yolk Sac edge
      2. Grows at 1 mm per day
      3. Fetal heart activity by Crown-Rump Length 5 mm
   4. Crown-Rump Length most reliable dating measure
      1. Menstrual age (weeks) = Crown-Rump Length (cm) + 6.5
      2. Table of menstrual ages per Crown-Rump Length
         1. 6.1 Weeks: 0.4 cm
         2. 7.2 Weeks: 1.0 cm
         3. 8.0 Weeks: 1.6 cm
         4. 9.2 Weeks: 2.5 cm
         5. 9.9 Weeks: 3.0 cm
         6. 10.9 Weeks: 4.0 cm
         7. 12.1 Weeks: 5.5 cm
         8. 13.2 Weeks: 7.0 cm
         9. 14.0 Weeks: 8.0 cm
2. Second and third trimester
   1. Fetal Biparietal diameter
   2. Fetal Head Circumference
   3. Fetal Abdominal Circumference
   4. Fetal Femur Length
   5. Composite Fetal Age
      1. Simple average (regression formulas)
      2. Best combination for accuracy
         1. Fetal Head Circumference
         2. Fetal Femur Length
3. References
   1. Paspulati (2004) Radiol Clin North Am 42(2):297-314