MEASUREMENT OF GESTATIONAL AGE

1. Last Menstrual Period (LMP)
2. Mean Sac Diameter = average diameter of gestational sac measured in 3 planes
• Measure chorion tissue/fluid interphase.
• Do not include the decidual reaction or chorion.
• MSD + 30 = menstrual age in days.
• MSD is the best measurement in £ 8 weeks when the fetus is flexed.
• Normal gestational sac size versus gestational age (Table / Graph).

3. Crown Rump Length (CRL) = longest measurable length of the embryo or fetus excluding the lower limbs, yolk sac and umbilical cord.

• Measurement.
• Crown (middle of midbrain) to embryonic or fetal rump.

• From 6-8.5 menstrual weeks the maximum length of the embryo is not accurately represented by the CRL as normal flexion of the embryo results in the middle of the midbrain being located below the highest point of the head. As the pregnancy progresses, embryonic head extension makes the middle of the midbrain the highest point.

Best measurement is greatest length and not CRL.

• Data derived from women who underwent in vitro fertilization demonstrated that CRL growth is curvilinear and not linear (1).
• This is the best parameter for early dating of pregnancy.
• Variability = ± 8% due to:
• Normal biologic variation in fetal size.
• Variation in times of ovulation and fertilization.
• Errors in measurement and technique.
• 95% confidence interval (CI) in pregnancy dates for CRL is 2-3 days prior to 11 weeks and 2-5 days after 11 weeks (2).
• Normal Table – Robinson et.al. 1975.
• Crown rump length versus gestational age (Table) – Hansemann et.al. 1979
• Crown rump length versus gestational age (Graph)– Hansemann et.al. 1979

REFERENCES

1. Daya S. Accuracy of gestational age estimation estimation using fetal crown-rump measurements. Am J Obstet Gynecol 1993;168:903-908.
2. Degani S. Fetal biometry: Clinical, pathological and technical considerations. Obstetrical and Gynecological Survey 2001;56(3):159-167.