What are Timed Pregnant Rodent Models?

What are Timed Pregnant Mice? 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.

Embryonic Diapause in Timed Pregnant Mice and Rats

Several methods are used to determine gestational age including development, weight, size, and appearance, however litter size influences these factors. An excellent reference is available online at UNSW for rats and mice.

Development of embryos and fetuses typically proceeds along a well-defined timeline, researchers occasionally observe that a harvest appears to consist of embryos or fetuses younger than the known gestational length would suggest.

Embryonic diapause may be the likely culprit in those instances when embryos or fetuses appear younger than expected. Embryonic diapause is a temporary halt in embryonic development which occurs prior to blastocyst implantation. This can be induced by environmental stressors or maternal input such as lactation1. As a result, the actual rate of embryo development can vary from the nominal gestational age.

Preferred Strains and Stocks for Timed Mating

ModelBreeding AgeLitter SizeTypical Gestation
Swiss Webster8 weeks11 pups19 - 20 days
Black 68 weeks7 pups18 - 20 days
Sprague Dawley®8 - 10 weeks10 pups21 - 23 days


The excellent reproductive performance of the Sprague Dawley® model makes it a preferred choice for generating timed pregnant female rats. Embryos from timed pregnant Sprague-Dawley® rats are used in the study of embryonic development, teratology, and reproductive toxicology, as well as in experiments using fetal tissue from specific embryonic ages.


The outbred Swiss Webster mouse also has superior reproductive performance and is an excellent choice for experiments in which an outbred mouse model is acceptable. If an inbred strain is required, the Black 6 mouse is commonly used. Timed pregnant Swiss Webster mice will typically provide greater embryo/fetus yields compared to the Black 6, which has smaller litter sizes.

Applications of Timed Pregnant Mice and Rats

Timed pregnant animals are used extensively in genetic engineering, developing tissue cultures, and developmental research. The ability to harvest embryos and fetuses at particular gestational stages is critical for these studies.

  • Embryo harvest (usually following superovulation and timed mating) is a critical step in the creation of genetically modified animal models. Zygotes are typically used for pronuclear microinjection (including CRISPR gene editing), whereas blastocysts are harvested at a later time point and used as donor embryos for injection of modified embryonic stem cells.
  • Mouse and rat embryos and fetuses are commonly used to generate primary tissue cultures, such as neuronal cultures or mouse embryo fibroblasts.
  • Timed pregnant mice and rats are commonly used to study the impact of diet or other inputs during pregnancy and on the resulting progeny.
1. Lopes, F. L.; Desmarais, J. A.; Murphy, B. D. Reproduction 2004, 128 (6), 669-678.

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