Pigs tend to produce large litters in a relatively short period of time. A sow stays pregnant an average of 114 days, although it's not uncommon for pigs to lose their babies before they are born, or have one or more stillborn piglets. She might not look pregnant until about three weeks before she delivers, but that doesn't mean there's a problem with the piglets.
An easy way to remember the length of gestation in swine is the rule of threes: three months, three weeks and three days, for a total of 114 days. This is an average length; pigs born a couple of days early or late should be healthy. The gestation lengths can vary with each pregnancy, so don't worry if your sow delivers at exactly 114 days for one pregnancy and 116 days for the next pregnancy.
The 30-day mark is an important one during swine pregnancy. That's about the time the piglets' bones are beginning to develop. If there's a problem with the pregnancy before 30 days that causes the piglets to die, the mother's body will reabsorb the babies and she should resume her normal heat cycles. After 35 days, the babies can't be reabsorbed completely because their bones have begun to calcify. Instead, the sow will deliver mummified bodies of her babies.
How to Determine Pregnancy
Keeping track of your sow's cycle is the easiest way to determine whether she's pregnant. When she's in heat and is not bred by a boar, her next heat cycle will occur within 21 days in most cases. Note the date she's bred, and if she doesn't start another heat cycle in about three weeks, it's likely she's pregnant. She won't look pregnant until much farther along in the pregnancy. If you think she's pregnant but she goes into heat, ask you vet whether your sow needs an ultrasound to confirm she's miscarried.
Time to Deliver
Because you can't predict the exact day your sow will deliver, watch her for signs she's ready. She'll start nesting, or scooting around the straw or other bedding in her area to make a warm and soft place to deliver. Her belly might drop so that it appears almost to touch the ground, and her vulva will swell. You also might notice dark discharge, which is a sign delivery is imminent.