Development of the surgically produced singleton mouse fetus.

All but one mouse embryo were removed from uteri on Day 8 of gestation and delivered via cesarean section on Day 18. Singletons were compared on various developmental indices to similarly delivered controls permitted to reside in uteri containing the normal complement of fetuses. The former were heavier at birth and weaning, had smaller ano-genital distance ratios (ano-genital distance/delivery weight), and displayed walking, gripping, eye-opening, and vaginal-opening earlier than controls. Singletons also exhibited greater durations and frequencies of various prenatal activities relative to controls when observed on Day 17 of gestation. Singleton status did not advance the onset of prenatal activity.