The relationship of a newly described acute-phase protein to human gestation.

Rho is a newly described acute-phase serum protein which increases in titer in response to a variety of inflammatory and infectious diseases. It is present in 100 per cent of sera from pregnant women by the twentieth week of gestation, rises in titer until term, and then falls slowly during the postpartum period. It was barely detectable in some fetal sera and in some amniotic fluid samples near the end of the first half of gestation, and it could not be demonstrated in fetal tissue obtained at that time. Fetal serum titers increase with gestational age, but fetal levels remain below those of maternal sera. Undifferentiated placental tissue and estrogen/progestin do not appear to evoke rho antigen production. The stimulating mechanism during gestation remains unknown.