Pigs can become pregnant from around 18 months of age. Still-births and early failures are common in pigs, so you may notice a slight change in behavior that doesn’t lead to a pregnancy. If the embryos remain viable beyond 10 days, then the signs of gestation slowly begin to emerge. Pigs show few physical signs of pregnancy until late on, but subtle changes in behavior can indicate that a pig is expecting a litter.
The gestation period of a pig is around 114 days. During this time, the pig will undergo a few physical changes, but these don’t become noticeable until toward the end of the pregnancy. Therefore, the most reliable means of pregnancy detection, aside from a veterinary exam, is a failure to return to the estrous cycle. If the sow is not pregnant, she will return to her estrous cycle 17 to 21 days after mating. Naturally, if you are unaware of whether or not she has mated, for example, if she’s been left alone with boars while in heat, you will need to check for signs of menstruation.
Early Stage Physical Changes
The physical signs of pregnancy don’t present themselves until around 3 months of gestation. At this time, sow will have a larger than normal abdomen, giving her a pot-bellied appearance. These are the earliest physical signs, aside from failure to menstruate. During gestation, the sow’s heartbeat will deviate from its regular pattern. This will only be apparent if you’re regularly measuring her vital signs. Commercial breeders may elect to monitor their sows this closely, but it is not typically essential.
Changes in Behavior
Once a sow becomes pregnant, she will not have any interest in reproductive behavior. She will ignore the advances of any males and, combined with her failure to menstruate, will generally lose all of her sexual behaviors, such as making courtship-related sounds and gestures. The sow will also display a heightened appetite during the final few weeks of the pregnancy because her body stops producing fat and all existing fat is converted into milk. If you are responsible for feeding, increase her food intake at this stage and introduce lots of vitamin A, in the form of fruit and vegetables.
Late Stage Physical Changes
At around 3 months in she will begin to change in appearance quite dramatically. She will have swollen udders, a swollen vulva and a distinct pot belly. The altered appearance of the belly can be quite dramatic because the loss of fat on the rest of the body contrasts with the distended abdomen.
The Final Stages
The period before delivery is called farrowing. At this stage, her behavior will alter dramatically, as birthing instincts begin to take hold. She’ll exhibit nesting behavior, characterized by gathering up straw and forming it into a nest. Ensure she has a soft place to rest, with plenty of nesting material. Restlessness is common, as is a small amount of blood discharged from the vulva, straining and possible whining or groaning. When she’s ready to deliver her litter -- typically consisting of between 8 and 12 piglets -- she’ll lie down and cease all other activity.
Simon Foden has been a freelance writer and editor since 1999. He began his writing career after graduating with a Bachelors of Arts degree in music from Salford University. He has contributed to and written for various magazines including "K9 Magazine" and "Pet Friendly Magazine." He has also written for Dogmagazine.net.