When does a pregnant rat's belly start to enlarge? - briefly
Visible abdominal swelling becomes noticeable around gestation day 10–12, when embryos enlarge and the uterus expands. Before this stage, the abdomen remains indistinguishable from that of a non‑pregnant female.
When does a pregnant rat's belly start to enlarge? - in detail
The gestation of a laboratory rat lasts approximately 21–23 days. Uterine expansion begins shortly after implantation (around day 4–5), but the abdomen remains indistinguishable from a non‑pregnant female until the embryos reach a size that pushes the uterine walls outward.
Visible abdominal swelling typically becomes detectable between days 10 and 12 of pregnancy, when the combined mass of the embryos and placental tissues starts to lift the dorsal surface of the abdomen. By days 14–16 the increase is pronounced; the belly appears rounded and the fur over the ventral region may become stretched.
The onset and degree of abdominal enlargement depend on several variables:
- Strain: Larger‑bodied strains (e.g., Sprague‑Dawley) show earlier and more obvious swelling than smaller strains (e.g., Wistar).
- Litter size: Larger litters generate greater uterine volume, accelerating visible expansion.
- Maternal condition: Obese or heavily conditioned females may mask early swelling, delaying detection.
In practice, researchers confirm pregnancy by palpation or ultrasound around day 10, then monitor the belly’s growth through day 16 to assess normal progression.