Timed pregnant mice and rats are important research tools, with applications in developmental research, genetic engineering, and generating tissue cultures. They are the most efficient way to obtain a cohort of embryos, fetuses, or pups with a precisely defined, matched age. But what exactly are timed pregnant mice?
What Are Timed Pregnant Mice and Rats?
A timed pregnant female is a mouse or rat with a precisely known mating date. This permits either the harvest of embryos or fetuses at a desired gestational age, or production of litters born on a desired day.
These timed matings are accomplished by setting up matings on a specific day and checking for evidence of copulation the next day. Females which show evidence of copulation (sperm plugs in mice, positive vaginal smears in rats) are isolated. This pinpoints mating — and presumably conception — to a particular twenty-four-hour period.
At Taconic Biosciences, the day of sperm cell detection in females is called the "sperm-positive date" and is considered Day One of gestation (All timed pregnant models should be ordered by the sperm positive date).
During the early stages of pregnancy, it is difficult to confirm pregnancy by visual observation or palpation. At fourteen days, pregnancy can be confirmed by visual observation and palpation of in utero pups.
Key Takeaways
1. Lopes, F. L.; Desmarais, J. A.; Murphy, B. D. 
