Amniotic fluid accumulation in the first trimester.

Amniotic fluid volume was estimated sonographically in 507 of 977 first trimester pregnancies with living fetuses and in 99 of 227 instances of missed abortion. Volume is correlated with fetal length, cross referenced by average heart rate for living fetuses. Fluid volume increases exponentially after 9 weeks, consistent with the onset of renal function. Fluid volume tends to be excessive with missed abortion. Free fluid volume is a physiologic variable, determined from morphologic features. Standards are proposed for amniotic fluid volume dynamics in the first trimester.