The effect of restraint stress in early pregnancy in mice.

Mice were exposed to 5 h of restraint stress on Days 1-3, 4-6, or 1-6 of pregnancy in the morning (08:30-13:30 h, a.m.) or afternoon (13:30-18:30 h, p.m.). Stress reduced the pregnancy rate from 90 to 52% (P less than 0.005) and average litter size on Day 18 from 8.2 to 5.2 young (P less than 0.005). Stress for 6 days was more effective than for 3 days (P less than 0.005) and an a.m. stress was more effective than a p.m. stress (P less than 0.005) in reducing the average litter size. Animals examined on Day 7 after 6 days of a.m. stress had decreased numbers of normal corpora lutea (CL), increased numbers of abnormal CL, decreased serum progesterone concentrations and tended to have fewer implantation sites. Abnormalities of embryo transport and implantation were also present. Changes in CL morphology and embryo transport and development were evident on Day 4 after only 3 days of restraint stress. These results show that many reproductive events of early pregnancy can be disrupted by restraint stress.