The levels of HCG in the serum and urine of post-partum women have been measured by RIA over a period of several days. The rate of disappearance of HCG from the serum decreased with time, resulting in an apparent progressive increase in the half-life of HCG during the first 3 days. The resulting composite disappearance curve appeared to be the result of 2 linear components, the first corresponding to a half-life of 8.9 hr and the second to a half-life of 37.2 hr. Similar half-life values were found for HCG present in newborn infants (9.1–16.8 hr) and for HCG injected into one nonpregnant subject (32 hr).
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MIDGLEY et al. (1968) studied this question.