Confirming pig pregnancy quickly after breeding saves money and time — open sows can be rebred or culled rather than fed for weeks under the assumption that they're pregnant when they aren't. Swine operations with tight farrowing schedules can't afford to find out at week 12 that a sow never conceived. Here's what to look for and when.
Once pregnancy is confirmed, use the pig gestation calculator to set the expected farrowing date from the breeding date.
The Earliest Sign — Absence of Return to Estrus
The pig estrus cycle runs 18–21 days, with most sows cycling every 21 days. The first signal of possible pregnancy is a sow that does not come back into heat at 18–21 days post-breeding. In practice, this means watching for the signs of estrus — a swollen, reddened vulva, the standing heat response (remaining motionless when pressure is applied to the sow's back), and mounting activity from herdmates — and noting their absence at the expected return date.
This is a useful filter but not a confirmation. Some pregnant sows show apparent estrus early in pregnancy. Some open sows may temporarily stop cycling due to stress, poor nutrition, or seasonal anestrus. The absence of heat at day 18–21 narrows the probability of pregnancy — it doesn't confirm it.
Mark the calendar twice: day 18 and day 21 after breeding. Watch for standing heat during the "boar exposure" check — walking the boar along the pen front and watching for sow response — on both days.
Ultrasound — Earliest Confirmation
Transabdominal ultrasound is the standard for early pig pregnancy confirmation in commercial operations. A portable ultrasound unit with a 3.5 MHz probe placed against the sow's flank (behind the last rib, angling slightly forward) can visualize embryonic fluid sacs from approximately:
- Day 18–20: Earliest possible detection in experienced hands
- Day 21–25: Reliable detection by trained farm staff
- Day 28+: Highly reliable; fetal heartbeats visible
Handheld swine ultrasound units from brands like Renco or IMV Imaging are widely used on commercial farms. Pregnancy checking at day 21–25 is standard protocol in tightly managed farrowing operations. Results are immediate, no lab is required, and false positives are rare in experienced hands.
A positive finding is a cluster of black fluid-filled spheres (embryos surrounded by amniotic fluid) within the uterine horns. A negative result shows only uterine tissue with no fluid structures.
Progesterone Testing
Blood or saliva progesterone tests are a laboratory-based confirmation method. Pregnant sows maintain elevated progesterone levels (above 2 ng/mL in blood) beyond day 21, while open sows see progesterone fall sharply as they return to the follicular phase before their next estrus.
Progesterone testing is less commonly used than ultrasound because it requires lab processing and doesn't provide as early or as immediate results. It's useful in specific situations — confirming pregnancy in sows where ultrasound access is limited, or in small herds where a single vet blood draw is more practical than purchasing ultrasound equipment.
Physical Signs — What to Watch For Through the Pregnancy
The physical changes that a non-expert can observe without equipment don't appear until well into the pregnancy:
Weeks 1–9: No visible external signs. The sow looks and behaves normally. Belly size is unchanged. The only behavioral shift some people notice is a slight increase in appetite and a tendency toward calmer behavior — but these are subtle and unreliable as indicators.
Weeks 10–13: Abdominal girth begins to increase. The belly looks wider when the sow is viewed from behind. In thin sows, the enlargement may be more obvious than in fat sows. The mammary glands (teats) along the belly begin to enlarge slightly.
Weeks 14–16: Belly enlargement is pronounced and obvious. The udder fills with milk. The sow slows down, eats more, and spends more time lying down. In the final week, restlessness increases.
Pre-Farrowing Signs — The Final 24–48 Hours
The days immediately before farrowing produce the most reliable and obvious signs:
- Udder filling: The teats enlarge and the udder becomes firm to the touch 24–48 hours before farrowing
- Milk letdown: You can express a few drops of milk from the teats approximately 12–24 hours before farrowing — this is one of the most reliable pre-farrowing indicators
- Nesting behavior: The sow roots at bedding, pushes material around, and appears to be building a nest — a deeply instinctive behavior regardless of whether bedding material is present
- Vulva changes: The vulva swells, reddens, and may have a mucus discharge as cervical dilation begins
- Restlessness: The sow repeatedly stands, lies down, and repositions — this can start 12–24 hours before farrowing and intensifies as active labor begins
Most sows farrow within 12 hours of visible nesting behavior. Move sows to the farrowing pen at least 5–7 days before the expected farrowing date so they can acclimate — moving a sow in active pre-labor is stressful.
Sources & References
- Penn State Extension — Swine reproduction and pregnancy detection
- Merck Veterinary Manual — Porcine reproductive management
- University of Minnesota Extension — Swine gestation and farrowing