“…This work emerged from ongoing discussions over several years among the authors regarding the basis of malformations and disruptions in twins. These diverse disorders were relatively easily separated into two distinct groups, the first with primary malformations of presumed early embryonic origin, and the second with structural defects variably defined as malformations or disruptions, attributed to prenatal defects in vascular perfusion or hemorrhage (Cantrell, Haller, & Ravitch, 1958; Duncan, Shapiro, Stangel, Klein, & Addonizio, 1979; Gulczyński, Świątkowska‐Freund, Paluchowski, Hermann‐Okoniewska, & Iżycka‐Świeszewska, 2019; Quan & Smith, 1973; Rollnick, Kaye, Nagatoshi, Hauck, & Martin, 1987; Steiner et al, 2018; Van Allen, Curry, & Gallagher, 1987). Here we consider the first group of recurrent constellations of embryonic malformations (RCEM), which includes limb–body wall complex (LBWC), Mullerian duct aplasia‐renal anomalies‐cervicothoracic somite dysplasia (MURCS), oculoauriculovertebral spectrum (OAVS), omphalocele‐exstrophy‐imperforate anus‐spinal defects (OEIS) complex, pentalogy of Cantrell (POC), vertebral‐anal‐cardiac‐tracheoesophageal fistula‐renal‐limb (VATER/VACTERL, hereafter VACTERL) association (Cantrell et al, 1958; Duncan et al, 1979; Keppler‐Noreuil et al, 2007; Quan & Smith, 1973; Rollnick et al, 1987; Van Allen et al, 1987), and possibly others.…”