Specimen: maternal serum, usually during weeks 16 to 18 week of gestation, although testing may be done from 15 to 20 weeks.
Units:
(1) reported in IU/L
(2) normally maternal serum HCG is 20,000-40,000 IU/L at time of screening
multiple of the mean = MoM =
= (average of maternal serum HCG determinations) / (median of the normal reference interval at the testing laboratory for stated gestational age in weeks)
Correction for maternal weight:
(1) since the maternal plasma volume changes with body size, a given amount of HCG will vary proportionate to body weight.
(2) correction factor = 10 ^ (0.2493
- (0.003792 * (maternal weight in pounds)))
(3) this is divided into the uncorrected MoM
(4) in women with high body weights, the adjustments may be too high resulting in false positive findings. The cut-off for making the adjustment varies, with 200, 250 or 280 pounds used as the upper limit for adjustments (200 pounds is used in the spreadsheet).
(5) in women with low body weights, the lowest weight used for the equation is 90 pounds
Correction for Black race:
(1) correction factor = 1.07
(2) the correction factor is divided with the uncorrected MoM
Correction for insulin dependent diabetes:
(1) values for mothers with insulin dependent diabetes mellitus are 5% lower
(2) correction factor = 0.95
(3) the correction factor is divided into the uncorrected MoM.
Correction for multiple gestations:
(1) in twin gestations, divide 1.84 into the uncorrected MoM
(2) correction factor for triplets not given
Interpretation:
• in Down's syndrome the maternal serum HCG is increased, typically with MoM 2.04
• low HCG MoM (<= 0.5) can be seen in trisomy 18
• very high HCG MoM (> 3 MoM) can be seen in hydatidiform mole and pregnancies with multiple fetuses
