Pseudopregnancy

Content sourced from Wikipedia, licensed under CC BY-SA 3.0.

Pseudopregnancy is a false pregnancy. A female shows signs of pregnancy, such as nesting, mammary development, and lactation, but there is no fetus. The key driver is the corpus luteum, a structure that forms after ovulation and keeps producing the hormone progesterone, which supports maternal behaviors. In many species, the corpus luteum disappears without a pregnancy, but in some it stays active and creates a true-looking pregnancy without an embryo.

In most animals, signals from an embryo (like certain hormones) tell the body there is a pregnancy and help keep the corpus luteum going. Some species may not rely on those signals as much, so the corpus luteum persists even without a fetus.

Cats
Cats are induced ovulators, meaning they only ovulate after mating. If fertilization doesn’t happen, the corpus luteum can stay for about 37 days (roughly 41 days on average), causing pseudopregnancy. Signs are usually mild, and behavior or mammary changes are not always obvious. The hormone pattern differs from normal pregnancy: estradiol rises after mating and then falls, while progesterone rises a few days after copulation, peaks around day 21, then declines. Pseudopregnancy in cats is not very common and lasts about half as long as a real pregnancy.

Dogs
Pseudopregnancy is a normal, common process in dogs. It usually occurs about 45–60 days after a heat (estrus) period. After ovulation, progesterone rises; pseudopregnancy happens when progesterone falls and prolactin rises, driving mothering behaviors, nesting, and milk production. Spaying a dog near the end of estrus can acutely induce or prevent pseudopregnancy by removing the source of hormones. In dogs, mammary development and lactation are common during pseudopregnancy.

Other species
- Rodents (rats and mice): The nocturnal surge of prolactin activates and maintains the corpus luteum, creating pseudopregnancy. There are two induced forms: coitally induced (triggered by mating) and non-coitally induced (triggered by social or environmental cues). Mating with an infertile male can cause the corpus luteum to persist and produce pseudopregnancy signs like mammary development and nesting.
- Pigs (sows): Pseudopregnancy can happen when embryos are resorbed after mating, leading to long periods of anoestrus and signs of pregnancy without piglets. Pregnancy diagnosis is important for breeding efficiency. Mycotoxins can also cause pseudopregnancy by interfering with normal hormonal cycles.
- General notes: In many species, the presence of an embryo provides signals to maintain the corpus luteum. In dogs, researchers think progesterone may be maintained without embryo signals, so the corpus luteum can persist even without pregnancy in some cases.

In short, pseudopregnancy is a normal, hormone-driven state in which females exhibit pregnancy-like changes without an actual pregnancy. It varies a lot by species and can often be managed or prevented by hormonal or surgical treatment when necessary.