Gestation

Pregnancy usually lasts approximately 22.5 days from mating to birth, so a mating on the evening of day 0 would produce a litter around midday of day 23. If the litter is conceived directly after the birth of a previous litter the gestation period may be extended by several days.

If a problem occurs with the pregnancy, the foetuses may be reabsorbed by the female. It is not always possible to distinguish reabsorption from phantom pregnancy.

Rats are also able to suspend implantation of the fertilized egg if conditions are not ideal at the time of mating.