The presence of a meconium-like substance in second-trimester amniotic fluid.

Previous studies of the significance of meconium-stained second-trimester amniotic fluid have relied upon visual assessment of fluid color as the screening procedure followed by spectrophotometric analysis at 405 nm to confirm the presence of "meconium" (incidence of 1% to 3%). This assumes that in the absence of discolored fluid there is no "meconium." In the present study, prospective spectrophotometric analysis was performed on 123 serially obtained amniotic fluid specimens sampled at 15 to 19 weeks' gestation. The presence of an absorbing species peaking near 405 nm was observed in 91% of the samples. This substance is generally not present at term. Its presence does not correlate with pregnancy outcome and is of no prognostic significance. However, it is related to gestational age and placental implantation site. Further analysis supports the hypothesis that our tracings represent a variety of pigments derived from the metabolism of intra-amniotic hemoglobin resulting from the normal physiologic development of early pregnancy.