The rate of growth of one portion of the body with respect to another may be
expressed as various ratios. By comparing the growth one parameter with
another, one may be alerted to a variety of fetal conditions ranging from IUGR
to skeletal dysplasias to fetal triploidy (1).
HC/AC (Ratio of
Head Circumference to Abdominal Circumference) Tables
- Used between 15-42 weeks.
- Normal ratio:
- <36 weeks = 1:1
- >36 weeks - ratio
decreases as the AC increases.
- This ratio helps to differentiate
between symmetric IUGR, asymmetric IUGR and a normally proportioned fetus.
- Symmetric IUGR - ratio is
normal.
- Asymmetric IUGR - ratio
increases as the head size is maintained at the expense of the AC
("Brain-sparing effect").
FL/BPD (FEMUR LENGTH TO BIPARIETAL DIAMETER RATIO)
|
- Normal ratio = 79 ± 8% (2).
- Increased ratio
- Microcephaly.
- Technically poor
measurement of the femur that erroneously included the distal femoral
epiphysis).
- Decreased ratio
- Large head.
- Skeletal dysplasia
(this ratio is a good indicator of fetal dwarfism as there is a very low
chance that a normal fetus will have a value that falls outside ±4 SD of the mean (1).
FL/AC (FEMUR LENGTH TO ABDOMINAL CIRCUMFERENCE RATIO) (3)
|
- Normal = 22 ± 2% (constant after 24 weeks).
- When ratio falls <20% or
>24%, the AC should not be used in the assessment of gestational age.
- Growth retardation = >24%.
- Macrosomia = <20% (3).
- Crang-svalenius E, Jorgensen
C. Normal ultrasonic fetal growth ratios evaluated in cases of fetal
disproportion. J Ultrasound Med 1991;10:89-92.
- Hohler CW, Quetel TA.
Comparison of ultrasonic femur length and biparietal diameter in late
pregnancy. Am J Obstet Gynecol 1981;141:759-762.
- Hadlock FP, Harrist RB,
Fearneyhough TC et.al. Use of femur length/abdominal circumference ratio
in detecting the macrosomic fetus. Radiology 1985;154:503-505.