The Stages of Gestation of Domestic Pigs

Pig gestation timing starts at breeding. Most sows have estrous cycles lasting 21 days, so you can try to breed them every three weeks. They stay in standing heat, the the period when a sow is willing to breed, for only 36 to 60 hours, typically. Note the date you put the sow and boar together for breeding; this is the day you start counting the gestational time. She won't show immediate signs of pregnancy -- the simple way to determine if she's pregnant is to see if she goes into heat again in three weeks. If she doesn't, she's likely pregnant.

It takes a while for fertilized eggs to make it to their final resting place in the sow's uterus -- usually between 15 and 20 days. The embryos remain viable only if more than four of them successfully implant; otherwise, the sow's body reabsorbs the embryos and resumes a normal estrous cycle. Five or more implanted embryos mean the sow's body will release enough estrogen to maintain the pregnancy.

Until day 30, the fetal skeletons are soft and tissuelike. If a problem occurs with the sow's pregnancy up to this day, her body reabsorbs the piglets instead of allowing her to deliver stillborn babies. After 30 days of pregnancy, the piglet bones begin to calcify and harden. This is key to their development and the pregnancy's success. After about day 35, pregnancy problems might lead to the sow delivering mummified stillborn babies. After the skeleton forms, the babies look like tiny versions of piglets, but they aren't quite ready for birth. At 60 days, they develop much of their immuno-competence, using antibodies from the sow. They spend the rest of gestation growing and finalizing development on their organs, systems and brains.

Even when using the 114-day guideline, you won't know exactly when the piglets will appear. About four days before babies appear, you might notice your sow has a swollen or red vulva. The sow might start creating a nest in her stall about 24 hours before she delivers, moving bedding into a soft pile. You also might notice fluid leaking from her teats up to 48 hours before the first baby arrives, turning from clear to more white as the delivery time approaches. Most sows don't require assistance to deliver, although you can help the piglets find a teat from which to drink colostrum as soon as possible. If you are concerned the sow is straining too hard without piglets appearing, call a vet for assistance.