Placental protein 12 (PP12): factors affecting levels in late pregnancy.

The circulating concentration of placental protein 12 was measured by radioimmunoassay in 109 pregnant women. The effect on the serum PP12 level of temperature, repeat freezing and thawing, and day-to-day and diurnal variation were assessed and the post partum changes of levels were studied. On average, PP12 levels in plasma are about one-half of those in serum in the same individual. PP12 immunoreactivity is destroyed by heating but not by repeated freezing and thawing. Changes in serum PP12 levels at various times of day showed a significant and consistent diurnal variation (F = 7.21; d.f.24; 96; P less than 0.001). The peak mean (+/- s.e.m.) value at 0800 h (80.8 +/- 8.7 ng/ml) was 41 per cent higher than the 24-h mean (P less than 0.05), and the nadir concentration at 1400 h (40.4 +/- 5.4 ng/ml) was 29 per cent lower than the 24-h mean (P less than 0.025). There is also considerable day-to-day variation (up to 72 per cent) in serum PP12 levels. If serum samples are taken at 0700 h, there is a slight negative correlation between PP12 concentration and placental weight (P less than 0.05), but not between PP12 level and birthweight of the child. In term pregnancy, the levels taken at 0700 h are higher (163.7 +/- 12.4 ng/ml) than at 0900 to 1300 h (115.7 +/- 11.4 ng/ml, P less than 0.001). The PP12 level is not affected by labour.(ABSTRACT TRUNCATED AT 250 WORDS)