Action of anti-progesterone monoclonal antibody in blocking pregnancy after postcoital administration in mice

Anti-progesterone monoclonal antibody injected intraperitoneally as a single dose 32 h post coitum completely blocked pregnancy in BALB/c and CBA nulliparous mice. The dose required was greater in CBA than in BALB/c females, but in both strains no implantation sites were detectable at autopsy on day 10 after mating. The action of the antibody in BALB/c females was associated with a failure to initiate an implantation response as indicated by the Pontamine Blue reaction. The most pronounced effect, however, was an arrest of embryonic development at a stage prior to cavitation. Plasma progesterone concentration in blood taken by cardiac puncture was greatly increased after treatment, by virtue of high-affinity binding by antibody in circulation. The results show that passive immunization against progesterone shortly after mating interferes with early hormone-dependent steps which are essential for normal embryonic development.