Disturbances that involve mitochondrial biogenesis (MB) and oxidative phosphorylation 90 (OP) underlie pathologic mechanisms leading to placental abruption (PA) -a complex 91 multifactorial disease characterized by premature separation of the placenta from the wall of the 92 uterus [1] . Genome-wide and candidate gene association studies have identified common 93 maternal single nucleotide polymorphisms (SNPs) in several MB and OP genes that are 94 associated with PA risk [2, 3] . However, findings were inconsistent, in line with other previous 95 reports of genetic associations in complex diseases [4, 5] . 96 Investigators have suggested that assessment of maternal-fetal genetic interactions and 97 assessment of imprinting effects, where risk is conferred depending on the parent-of-origin, may 98 explain the missing heritability of complex diseases, particularly those with perinatal origin such 99 as PA [4] [5] [6] . PA is a consequence of complex interplay of maternal and fetal genetics, 100 epigenetics, and metabolic factors. For instance, the fetal genome influences placental growth 101 and development, placental implantation and vascularization [7] , all of which have been related
Overall PAPE and PAGE study participants (cases and controls) were similar in socio-116 demographic characteristics and medical/obstetric history ( Table 1) . PAPE and PAGE PA-case 117 mother-infant pairs were similar to control mother-infant pairs with respect to maternal age, 118 marital status, employment, planned pregnancy, infant sex, alcohol use, drug use and vitamin 119 use. Compared to control mother-infant pairs, PA case mother-infant pairs were more likely to 120 smoke during pregnancy, have lower educational attainment, have lower pre-pregnancy body 121 mass index, deliver earlier (i.e., shorter gestational age), deliver infants with lower birth weight, 122 and have a diagnosis of preeclampsia in the current pregnancy. function [2, 27, 28] . PPARG (peroxisome proliferator-activated receptor gamma) belongs to the 197 PPAR-family of genes and is a master regulator of MB [29] . PPARG is highly expressed in the cells or act upon the myometrium to regulate contraction [32] , through pathways that involve 206 oxytocin receptor activation [33] . Preterm uterine contractions are associated with PA risk [34] . 207 We found strong evidence for imprinting effect of several imprinted genes (including 208 KCNQ1, NTM, and ATP10A) on PA risk. Imprinted genes may affect maternal-fetal interactions 209 that affect placental development [9, [35] [36] [37] . Imprinted maternal alleles are required for the 210 development of the embryo, and imprinted paternal alleles regulate formation of the placenta and the surrounding membranes of the embryo [8, 38] . KCNQ1 encodes a voltage-gated potassium 212 channel required for the repolarization phase of action potential [39] . KCNQ1 is expressed in the 213 placenta and implicated in embryonic and placental growth [36] . In mice, maternally inherited 214 target deletion of ASCL2, a gene that resides in KCNQ1 cluster, is lethal due to failure of 215 placental formation [37, 40] . The potential roles of both NTM and ATP10A in pregnancy-related Table 2 and Supplementary Fig 1) , and a network (Score=29) of 231 cardiovascular disease and free radical scavenging (Supplementary Table 2 and 232 Supplementary Fig 2) . Both of these networks align well with what is known about PA 233 pathogenesis [3] .
The underlying genetic architecture of PA has been examined in previous genome-wide 235 [2, 3, 12] and candidate gene association studies [2, 3, 12, 13, 47] , which reported predominantly 236 common, non-coding variants with modest effects and limited replication. Important strategies to 237 address subsequent missing heritability include family studies that assess gene-gene interactions 238 and imprinting effects [4, 5] . Family studies are advantageous because affected relatives are 239 more likely to share two nearby epistatic loci in LD that would be unlinked in unrelated studies and imputation of genome-wide data [5] .
Our study is the most comprehensive investigation, to date, of maternal-fetal genetic underlying mechanisms [48] . Our study was restricted to live births and interaction or imprinting 256 effects that contribute to stillbirth, a common complication of PA, may have been missed. We 
Study setting and study populations 270 The study was conducted among participants of the Peruvian Abruptio Placentae we applied a parametrization that introduces two interaction terms capturing incompatibility 356 between maternal and fetal genotypes; the interaction effects operate when the infant has one 357 copy and the mother has either zero or two copies of the risk allele. The Bayesian information criterion (BIC) was used for model selection. In addition to the maternal and fetal genotype 359 effects, we estimated the risk ratio (RR) of disease when the infant has one copy and the mother 360 has zero copies of the risk allele. The reference group is defined as mother-infant pairs carrying 361 zero copies of the risk allele.
Imprinting/parent-of-origin effect, which corresponds to the factor multiplying the 363 disease risk if the infant inherits a risk allele from the mother, was assessed using a likelihood 364 ratio test [2] . Bonferroni corrections were applied to correct for multiple testing: p-value<6.4e-4 365 for the 78 maternal-fetal interaction tests, and, p-value<1.8e-5 for 2,713 imprinting effect tests.
All analyses were conducted based on HWE, random mating, and rare disease 367 assumptions. Statistical analyses were conducted using PREMIM [24] , EMIM [24] , R (version 368 i386 3.1.2) and SAS (Version 13).