van Raaij et al developed equations for predicting body fat mass during pregnancy. One set is based on body density while the other uses total body water (TBW). The authors are from Agricultural University in De Dreijen, The Netherlands.
Patient selection: pregnant woman
Parameters:
(1) body mass in kilograms
(2) week of gestation
(3) body density in g/mL or 10^3 kg per cubic meter
(4) presence or absence of generalized edema
fat mass in kilograms =
= (body mass) / 100 * ((X / density) – Y)
X
No Edema or Leg Edema Only
Generalized Edema
10week gestation
496.4
451.6
20 week gestation
502.2
458.0
30 week gestation
510.8
467.5
40 week gestation
522.5
480.5
Y
No Edema or Leg Edema Only
Generalized Edema
10week gestation
496.4
451.6
20 week gestation
504.7
460.8
30 week gestation
518.1
475.7
40 week gestation
537.3
497.0
where:
• The X factor for the Siri equation is 495.
• The Y factor for the Siri equation is 450.
• The density of fat is normally 0.9 and the density of fat-free mass is 1.1.
• As a pregnancy progresses the density of fat-free mass declines as its water content increases.
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