Updated: Feb 19, 2019
When gerbils nurse their pups, hormones are released. These hormones can affect how long it takes the fertilized eggs to implant into the uterus.
Gerbils who have more than three pups in nest have significant delays in the birth of the second litter. This is why females who have only one or two pups give birth sooner than gerbils who have seven or eight pups in the nest.
There is a article called "Mating post partum and length of gestation in the Mongolian gerbil (Meriones unguiculatus)" by M. L. NORRIS & C. E. ADAMS that was published in 1981 in Laboratory Animals.
The chart is extracted from the data in the article.