Changes in blood urea nitrogen (BUN) concentration during pregnancy in the rat with or without obstructive uremia

Bilateral ureteral ligation changes the blood urea nitrogen (BUN) concentration in pregnant and non-pregnant rats. Marked increases of the BUN concentration occurred in both pregnant and non-pregnant specimens after ligation. The BUN concentration in sham-ligated rats did not change from day 16 through 19 of pregnancy, but increased on day 20 of pregnancy and thereafter. There was no significant difference in BUN between ligated pregnant rats on days 16 and 17 of pregnancy and ligated non-pregnant rats. As pregnancy progressed from day 18 onward, the BUN in ligated pregnant rats decreased when compared with that of ligated non-pregnant rats, but did not reach the level in sham-ligated pregnant rats. The BUN of mothers and fetuses both in the ligated and the sham-ligated groups were similar. Amniotic fluid urea nitrogen levels were higher than in blood on late fetal days. In ligated mothers, a significant relationship between maternal BUN and the number of fetuses was noted on day 22 of pregnancy. These results suggest that the fetal kidney becomes functional for excreting urea in late fetal life.