Mikheev et al. used a whole genome approach to identify major functional categories of genes whose expression depends on gestational age. Gene expression profiles were compared in the villous tissues of first (45-59 days) and second trimester (109-115 days) placentas from uncomplicated elective termination versus cesarean section term placentas. Subjects were excluded if they had any chronic disease, were on chronic medication, were smokers, or abused alcohol or drugs. Any differences in drug exposure for the termination or cesarean sections were not taken into consideration.

Tissue Condition Samples
Test chorionic villus third trimester pregnancy 4
Control chorionic villus first trimester pregnancy 4
Gene Fold Change (log2) Significance Genus Species