The concentrations of ovine placental lactogen (oPL) have been determined in maternal plasma, umbilical cord plasma, and allantoic fluid by an homologous radioimmunoassay for oPL which is sensitive to 0-1 ng hormone. Ovine placental lactogen was first detected in maternal plasma at 41-50 days of gestation and reached a peak concentration of 2547 +/- 226 (S.E.M.) ng/ml at 121-130 days in ewes with singleton gestations. The oPL concentration in cord plasma was 336-4 +/- 60-3 ng/ml and in allantoic fluid was 29-6 +/- 6-4 ng/ml. After surgical removal of the placenta, oPL disappeared from maternal plasma with a half-life of 29-1 +/- 1-3 min.
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Handwerger et al. (1977) studied this question.