A method for continuous monitoring of meconium in the amniotic fluid during labour

Abstract

In about 10% of pregnancies overall, the fetus discharges meconium (its bowel contents) into the amniotic fluid during labour. In about 10% of cases where meconium is passed, the fetus gasps, inhaling the sticky meconium into the upper respiratory tract. After birth, the meconium blocks the air passages in the lungs, impairing gas exchange — meconium aspiration syndrome (MAS). Up to 20% of infants suffering from MAS die and recently published studies have shown a long-term effect of MAS in causing cough and wheeze. The risk of meconium aspiration is thought to be increased by intrauterine hypoxia. At present, meconium is only noticed at birth or occasionally when amniotic fluid leaks past the presenting part of the fetus. A method has been developed which measures absolute meconium concentration with a 99% prediction interval of ± 30 gl⁻¹; allows monitoring of the rate of appearance of meconium linearly with a nonlinearity of 5%, and differentiates between meconium and blood. The method uses the ratio of the intensity of back-scattered light from the amniotic fluid at 700 and 415 nm, the latter being near the peak of light absorption by meconium and the former a reference value. The ratio is also affected by the presence of blood. However, blood has specific absorption peaks at 540 and 575 nm from which it can be detected (the presence of blood is also a significant abnormality, and is relatively uncommon). The measurement method could easily be integrated into an optical sensor mounted onto an intrauterine probe. The measurement of back-scattered light at 415, 540 and 700 nm would allow continuous monitoring of meconium which could provide valuable information for the study of the pathophysiology of meconium passage in utero during labour and allow preventative measures to be developed.