FETAL ASPHYXIA AND MECONIUM PASSAGE

The incidence of meconium staining of the amniotic fluid varies with gestational age:

• Rare before 26-28 weeks.
• ±15% at term.
• ³25% in the postmature fetus (>42 weeks).

Meconium passage occurs most commonly during the active phase of labor, less commonly at the start of labor and rarely on amniocentesis in the antepartum non-laboring patient. This suggests that there may be an association between uterine contractions and the passage of meconium.
The incidence is probably increased in some high-risk patients:

• Pre-eclampsia associated with fetal growth impairment.
• Less common in fetuses of diabetic mothers.

There is no experimental evidence to fetal lamb asphyxia with the passage of meconium. In the fetal lamb and monkey, meconium is present in the distal colon but is rarely expelled into the amniotic cavity even in the presence of asphyxial states of sufficient magnitude to cause fetal death.

Numerous studies by Manning and co-workers suggest that fetal asphyxia rarely, if ever causes meconium passage (1,2).

REFERENCES

1. Manning FA, Martin CB Jr, Murata Y et.al. Breathing movements before death in the primate fetus (Macaca mulatta). Am J Obstet Gynecol 1979;135:71.
2. Manning FA, Walker DW. Unpublished observations in: Manning FA (ed). Fetal medicine. Principles and practice. Appleton and Lange, Norwalk, Connecticut 1995:188-189.