First Trimester

• Mean sac diameter (MSD) is the preferred method for measuring gestational sac size

• The relationship between MSD and human chorionic gonadotropin should be used to aid assessment of the uterus and endometrial cavity in early pregnancy

• CRL should be used for gestational age assessment once a fetal pole us detected

• Discriminatory values for pregnancy viability in the 1st trimester:

• Fetal pole must be seen when MSD ≥ 18 mm (TV) or ≥ 25 mm (TA)

• Fetal heart activity must be seen when CRL ≥ 5 mm

Second and Third Trimester

• BPD, HC, AC and FL routinely obtained in 2nd and 3rd trimester

• BPD and FL should be used for gestational age assessment (HC may be substituted for BPD if technically more reliable)

Amniotic Fluid Index (AFI)

• Sum of the largest vertical depth of largest pool in each quadrant (note slim “slivers” of fluid, i.e. <1cm wide, should not be measured)
• Subjective assessment by an experienced sonographer/sonologist has been demonstrated to be as accurate as AFI.
• An abnormal AFI with a normal subjective assessment should be treated with caution
• AFI varies with gestational age, therefore refer to chart. Measurements below the 2.5% or above 97.5% are significant
• If formal AFI not possible, “deepest pocket” x3 approximates to the AFI

Cervical Length

• Length

• >3cm normal
• 2-3cm indeterminate
• <2cm abnormal
  

Fetal Head

• Cephalic index:BPD / OFD x 100; normal range: 70-86

• BPD correction formula √BPDxOFD/1.24 or BPD+OFD/2.25

• Head Circumference = (BPD + OFD) x 1.57

• Normal Nuchal Fold Thickness <6mm, 6mm or greater is considered a hard marker for chromosomal abnormality, valid to 20 weeks.

• Lateral ventricle Normal <10mm

Fetal Orbits

• 
  

Fetal Limbs

• 
  

Fetal Abdomen

• Abdominal Circumference = π x MAD

Fetal Renal Tract

• Fetal Renal Pelvic Diameter ( A.P.): normal < 5mm
• mild dilatation 5-7 cm, marker for Tr21, postnatal follow-up required
• moderate/marked ≥ 7 cm, marker for Tr21, requires referral to Perinatal Dysmorphology Group and 3rd trimester follow-up

Fetal Thorax

• Thoracic area to heart area ratio roughly 3:1

• Thoracic circumference measured around bony margins

Fetal Weight

• EFW should be reported as a 20 % +/- range rather than as a single weight, e.g. an EFW should be reported as being in “the range of 800 – 1200 gm” rather than as being “1000 gm +/- 20%”.

Nuchal Translucency

• Only valid between 45 and 82 mm

• Tr 21 risk should not be calculated from the NT in isolation.

• A NT above the 3.5 mm (99%) carries a significantly raised risk of fetal abnormality (especially cardiac abnormalities – prevalence 6%) and poor outcome regardless of the calculated Tr 21 risk (reference: http://www.fetalmedicine.com/11-14scanbook/Chapter2/chap02-7.htm,
  

Placenta Previa

• Prior to 26 weeks:
• Normal ≥ 2cm : report as not low-lying, do not state a measurement.
• low-lying if placenta < 2cm from internal os, report as low-lying with the measurement and recommend rescan at 34 weeks
  

• After 26:
• If ≥ 5cm from internal os: report as clear of the cervix
• f < 5cm: state distance from internal os in cm’s
  

• Avoid using grading systems to describe relationship of the placenta to the internal os, rather the shortest distance between the lower margin of the placenta and the internal os should be reported.

• The term placenta previa should be reserved for use in the third trimester only.

• Common reasons for incorrect measurements include an overfull bladder, Braxton-Hicks contractions and placenta obscured by fetal parts.

• Consider scanning with the bladder empty, translabial scanning and transvaginal scanning to better delineate the cervix and placental margin