Placental protein 5: circulating levels in twin pregnancy and some observations on the analysis of biochemical data from multiple pregnancy.

Blood was collected from 344 subjects with singleton pregnancy between 28 and 40 wk gestation, together with 70 samples from 24 subjects with twin pregnancy of comparable gestation. Circulating placental protein 5 (PP5) in singleton pregnancy rose from a median of 26 microgram/l at 28 wk to reach a plateau with a median of 38 microgram/l at 36 wk. The median PP5 levels in twin pregnancy were 50 microgram/l at 28 wk rising to 65 microgram/l at 38 wk. The ratio of the medians of PP5 levels in twin and singleton pregnancies fell from 1.94 at 28 wk to 1.62 at 38 wk. An "action line' for low levels of PP5 in twin pregnancy was defined by applying a multiple of the median for twin pregnancy.