Organ weight/bodyweight ratios: growth rates of fetal organs in the latter half of pregnancy with a simple method for calculating mean organ weights.

Abstract

Ratios for major organ weights compared with bodyweights of 1023 stillborn and liveborn babies who lived less than 72 h are presented. The ratios were calculated for 2 week increments of gestational age from 20 to 43 weeks and clearly depict the relative growth of fetal organs during the last half of pregnancy. The ratios for heart and for kidneys were virtually constant for the whole period of gestation examined. The ratios for thymus and spleen increased between 20 and 30 weeks gestation and then became constant, although the ratio for the spleen dropped slightly during the last 6 weeks. The ratios for liver, lungs and adrenals decreased between 20 and 30 weeks gestation, and then steadied. The ratio for brain declined very slowly throughout the period examined. An observation of practical importance was that all organ weight/bodyweight ratios were virtually constant after 30 weeks gestation. Approximate mean organ weight/bodyweight ratios between 30 and 43 weeks gestation were: heart 0.007, lungs 0.02, spleen 0.003, liver 0.04, kidneys 0.01, adrenals 0.003, thymus 0.004 and brain 0.13. By multiplying the mean ratio by the total bodyweight, the approximate mean weight for a particular fetal organ can be calculated in situations where charts of normal organ weights are not at hand.