Embryo-maternal dialogue starts shortly after fertilization and is exerted through both local and systemic signaling. We have discovered specific embryo derived pre-implantation factors (PIF), novel peptides that are secreted already at the two cell stage and which modulate cellular immunity. In the fallopian tube the embryo, a partial allograft, is tolerated by the mother. Embryo derived peripheral signaling (PIF) is also detected prior to implantation in maternal sera. This signal may also help prime the endometrium to facilitate implantation. Upon implantation, embryo-endometrial communication becomes direct and highly amplified. When the immune privilege appears to be secured, embryogenesis proper initiates. This requires proliferation/differentiation to be tightly controlled. Knowledge of proliferation promoters is ample while the factors involved in its control remain less understood. We have identified a class of novel proteins/peptides, developmental proteins (DPs), that are present in the embryo before a mature immune system has developed. Their role is to create a balance between pro and antiproliferative forces, to promote normal proliferation while controlling abnormal cell proliferation (i.e. due to carcinogens, toxins, viruses, and ionic radiation). DPs, may also redirect growth towards functionality through differentiation. DPs appear to act through a specific receptor negating growth factors action through promotion of tumor suppressors and inhibition of tumor promoters at 2 minutes, blocking DNA synthesis at ~24 hours, and promoting apoptosis at ~48 hours. When an embryo becomes incompatible with life, DPs may lead to growth arrest, PIF-like compounds decline, the immune system to be restored and the pregnancy is rejected. Final identification and use of PIF and DPs is likely to help both managing early pregnancy disorders and aid in treatment of proliferative disorders due to cancer and viral infection.