The amount and distribution of prenatal loss has been studied during the first pregnancy in albino rats of the random-bred, specific-pathogen-free colony maintained at Alderley Park.
Out of a total of 196 virgin female rats mated and found to have spermatozoa in the vaginal smear on the following day (Day 1 of pregnancy), fourteen (7·1%) failed to become pregnant. In 145 rats which became pregnant, the mean number of eggs shed was 12·2 ±0·19 and the mean number that implanted was 10·9 ±0·19. In eighty rats in which pregnancy was allowed to continue to the 18th to 20th day, the mean number of viable foetuses found at autopsy was 10·1 ±0·25. In various experimental groups, pre-implantation loss ranged from 9·8% to 13·9% of the eggs ovulated, the mean being 11·1% Post-implantation loss amounted to only 7·7% of the eggs ovulated, giving a total prenatal loss of 18·8%. It was found that the number of eggs ovulated was significantly related to body weight on the day of ovulation.
Out of 145 rats which became pregnant, seventy-nine (55%) showed some loss of eggs before implantation; 191 (10·8%) of the eggs were lost. Fifty-three per cent of this loss was found to be due to fertilization failure, and 43% was due to failure to develop into blastocysts. There was no correlation between the number of eggs ovulated and the number failing to develop into blastocysts, but there was a significant correlation between the number of eggs ovulated and the percentage lost before implantation. This effect, however, was confined to the individual uterine horn; the presence of large numbers of eggs in one horn did not influence the loss of eggs in the other.
In thirty-eight (55%) out of sixty-nine pregnancies some loss of embryos occurred after implantation and sixty-six (8·7%) embryos were lost. In no pregnancy in which implantation occurred were all the embryos lost. The number of eggs implanting in the uterus did not influence the percentage of embryos lost.
The mean diameter of implantation sites which survived to the end of pregnancy increased from 2·41 ±0·022 mm on Day 7 to 4·37 ±0·043 mm on Day 11 and 10·26 ±0·044 mm on Day 16. From examination of the remnants of implantation sites found in the uterus at autopsy an estimate could be made of the stages at which embryos died. Of the sixty-six lost after implantation, 73% died before Day 11, 20% between Days 11 and 13 and only 8% after Day 13. There was an indication, not reaching the level of statistical significance, that uterine crowding may have affected the amount of loss in the middle period, but it had no effect on loss in the early period.