About the Swine Gestation Calculator
Gestation in swine (Sus scrofa domesticus) lasts an average of about 114 days, commonly remembered by pork producers as the "3-3-3 rule": three months, three weeks, and three days. This calculator adds the gestation length to the date the sow was bred (natural service or artificial insemination) to estimate her farrowing (due) date. It also reports how many days remain, how many days she has been pregnant, and the current stage of gestation.
The formula
The math is a simple date addition:
Farrowing date = breeding date + gestation length
Using the species average, that is breeding date + 114 days. Days-remaining is the difference between the due date and today; days-into-gestation is the difference between today and the breeding date.
Why timing matters
Compared with cattle, sheep, or horses, swine gestation length is remarkably consistent — most individual sows farrow within a narrow window of about 111 to 120 days, with less breed-driven spread than other livestock. Knowing the expected farrowing date lets producers move the sow to a clean, prepared farrowing crate 3 to 5 days ahead of time, schedule pre-farrowing vaccinations, arrange for supervision around the clock as the date nears, and prepare supplies such as heat lamps and iron injections for the piglets.
Reference points
- A sow bred on June 1 using the 114-day average is expected to farrow around September 23 of the same year.
- The estrous cycle in swine repeats about every 21 days, so if a sow returns to heat roughly 18–24 days after breeding, she likely did not conceive and the date should be recalculated from the next successful service.
- Signs that farrowing is imminent — nesting behavior, udder filling, and restlessness — typically appear in the final 24–48 hours, regardless of exactly how many days have elapsed.