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In guinea-pigs ovariectomized before implantation, pregnancy can be maintained during the first 3 or 4 weeks, a critical period of early growth, by synthetic progestagens that do not metabolize to progesterone (norgestrel, medroxyprogesterone acetate and melengestrol acetate). It follows that peripheral plasma progesterone levels are related to placental progesterone secretion. There were indications that progesterone secretion by the placenta begins as early as day 16–18 of pregnancy. In the experimental animals the plasma progesterone levels, measured by a competitive protein-binding technique, were 9·4 ± 2·07 ng/ml (mean ± S.E.M.; n = 18) by days 20–22 of pregnancy; they continued to rise and reached 26·4 ± 3·88 ng/ml (n = 8) by days 27 to 29.

The metabolic clearance rate of progesterone measured in four pregnant ovariectomized animals with progesterone or progestagen implants, was < 10 l/day as in normal pregnant animals. The calculated production rates of progesterone in the experimental animals (0·1 to 0·35 mg/day) reflect the placental production rate of progesterone. A simple 'assay' is described for the high affinity progesterone-binding protein of pregnancy (progesteronebinding globulin).