DNA was isolated from placental tissue using the GenElute Mammalian Genomic DNA Miniprep Kit (Sigma, St. Louis, MO) with proteinase K and RNase treatment, according to the manufacturer’s instructions. Mitochondrial DNA copy number was estimated by comparing the abundance of the mitochondrial tRNALeu(UUR) gene (determined by quantitative RT-PCR, forward primer: 5′-CACCCAAGAACAGGGTTTGT; reverse: 5′-TGGCCATGGGTATGTTGTTA) and with that of the nuclear β2-microglobulin gene (forward: 5′-TGCTGTCTCCATGTTTGATGTATCT; reverse: 5′-TCTCTGCTCCCCACCTCTAAGT).