High resolution ultrasound can help streamline experiments requiring timed pregnant mice by determining the state of pregnancy, gestational age, and pregnancy losses. This protocol demonstrates how to assess mouse pregnancies and identify potential image artifacts that may mimic pregnancy. To begin, identify the bladder on the screen.
Scanning caudally from the bladder, identify the vagina. When the ultrasound transducer is positioned midline and low on the abdomen, the bladder and vagina are visible. Then scanning slowly and smoothly in a cranial direction, identify the bifurcation of the vagina into the left and right uterine horns.
Survey the left and right uterine horns. Up to mid-gestation, the mouse embryos will be positioned along the right and left peripheries. As they grow, the more distal portions of the uterus and their corresponding embryos will turn outwards and dorsally.
This protocol is especially useful when breeding genetically engineered mice. Typically, heterozygous with heterozygous crosses to yield homozygous offspring lead to failure of proper development, causing prenatal lethality. Mouse embryos at various stages of development are shown here.
Early stage mouse embryos, dead embryos, or resorbed embryos may resemble other organs in the abdomen or feces in the intestines, while intestinal loops may mimic the non-gravid uterus. Some investigators prefer to use a stage mounted transducer, such as shown in this image, but we prefer the handheld method of scanning, since this allows for fast and efficient imaging throughout the abdomen.
