Problem  An important subset of implantation defects/early abortion seems to be linked with a deregulation of the interleukin (IL)‐12/IL‐15/IL‐18 system as well as tumor necrosis factor (TNF)‐ and natural killer (NK)‐controlled/mediated networks at the decidual placental interface, both in case of deficient or excess expression. The presence of TNF in high amounts in the pre/peri‐implantation uterus and its pivotal role during pregnancy are difficult to reconcile with its abortive effects in ongoing pregnancy. We therefore searched for regulators of the IL‐12/IL‐18 family of cytokines as well as for antagonist(s) of TNF with potentially selective effects on implantation.Method of study  We first used Swiss mice to verify the presence in the murine reproductive tract of ‘new members’ of the IL‐12 family of cytokines, IL‐23 and IL‐27, as well as of tumor necrosis factor‐like WEAK inducer of apoptosis (TWEAK), described as acting with TNF as Yin and Yang of innate immunity in murine placenta/ decidua at days 0–12.5. We then compared expression by RT‐PCR in the CBA × DBA/2, and CBA × BALB/c murine mating combinations. Finally, we performed in vivo neutralization experiments of TWEAK and IL‐27.Results  Immunohistochemistry (IHC) studies showed that IL‐23, IL‐27, and TWEAK were expressed at the interface. For RT‐PCR, IL‐23 expression peaked at day 9.5 in the non‐aborting mating combination, a peak absent in the aborting one, and thus difficult to explain except by invoking a feed back on EB13 (Epstein–Barr virus‐induced gene 3 cytokine). Most important, an immediate post‐mating IL‐27 hyper expression was seen in the CBA × DBA/2 mating compared to CBA × BALB/c one. The difference in expression resurged and was statistically very significant by days 6.5–9.5, compatible with an early activation of inflammation on day 0.5 which would then peak again in the ‘resorption window’ where takes place the early NK/mph activation described by Baines et al. A significant TWEAK expression was present in both strains from days 0.5 to 4.5 peaking in both cases in the first days when it is known that intra uterine TNF also reaches high levels as a component of post‐mating inflammation. However, it was lower from day 1.5 in the abortion‐prone CBA × DBA/2 mating combination, and almost absent by days 6.5–9.5 when compared to the non‐aborting CBA × BALB/c mating combination. In both mating combinations, neutralization of TWEAK‐enhanced resorption rates, but surprisingly so did IL‐27 neutralization.Conclusion  TWEAK is likely offering protection against the deleterious effects of TNF in implantation explaining embryo survival in a TNF‐rich environment, and equal number of implants in both strains. However, there is a clear difference of protection in abortion‐prone mating peaking in the abortion/resorption window but starting early, and therefore possible links with the prevention of abortion in CBA × DBA/2 matings by interfering with complement activation as recently described by Girardi et al. are discussed, as well as consequences for our current view of feto‐maternal ‘seed and soil’ interplay. The apparently paradoxical effects of IL‐27 neutralization are also discussed.