A longitudinal study of leucocyte blood counts and lymphocyte responses in pregnancy: a marked early increase of monocyte-lymphocyte ratio.

Seventy-seven pregnant women were studied repeatedly from the first trimester until 4 weeks after delivery. Neutrophil count was increased but constant in all stages of pregnancy. In contrast, there was a marked increase in monocyte numbers in the first and early second trimester, followed by a gradual decrease towards normal before term. A progressive decrease in total lymphocyte count was observed up to 25-28 weeks, when it increased again, although not reaching the non-pregnant level before term. These fluctuations in monocyte and lymphocyte numbers have not been described before and could explain contradictory findings of previous studies. They may also be important in relation to the mechanisms which prevent rejection of the fetal allograft. Absolute T cell counts were not significantly decreased, but a striking reduction was observed in B cell numbers in early pregnancy. Lymphocyte response to T cell mitogens was decreased throughout pregnancy. It is concluded that immunological studies of pregnancy must take account of the dynamic nature of this condition.