Quick answer
The key early bovine milestones: PAG pregnancy proteins appear in blood from about day 24 (tests are usually run from day 28). Ultrasound becomes reliable around day 27 to 30, when the heartbeat is visible. Palpation starts at about day 35 to 40. Placentomes can be felt from roughly day 75 to 90.
The bovine gestation chart
The days above are for cattle. Buffalo and bison follow roughly the same early sequence, but the later stages stretch out over their longer pregnancies.
Pregnancy diagnosis by day
| Day | What becomes possible |
|---|---|
| 18–24 | No return to heat. It’s the first hint she might be pregnant, but not proof. |
| 24–28 | PAG proteins appear in blood. Commercial blood and milk tests are usually run from day 28. |
| 27–30 | Transrectal ultrasound becomes reliable. The embryo and heartbeat are visible by about day 30. |
| 35 | Amniotic vesicle can be palpated by an experienced vet. |
| 40 | Placental membranes can be felt; crown-rump length about 2 cm on ultrasound. |
| 55–90 | Fetal sex can be seen on ultrasound. |
| 75–90+ | Placentomes can be felt by rectal palpation. |
Bovine fetal size by month
Vets who palpate cattle carry a rough mental picture of fetal size. Charts vary a little from school to school, but this is the usual one:
| Month | Roughly the size of… | Placentome guide |
|---|---|---|
| 2 (≈ day 60) | a mouse | not yet felt |
| 3 (≈ day 90) | a rat | about a dime, 1–1.5 cm |
| 4 (≈ day 120) | a small cat | about a quarter, ~2 cm |
| 5 (≈ day 150) | a large cat | about a half dollar, ~3 cm |
| 6 (≈ day 180) | a beagle | 4–5 cm, starting to flatten |
| 7–9 | a large dog, then birth size (roughly 60–100 lb) | large and plaque-like |
A word of caution: placentome size is the least reliable way to age a pregnancy. One dairy study found it explained only about 40% of the variation in fetal age. Measurements like crown-rump length (early) or the width of the fetal head (later) are far better.
Crown-rump length in early pregnancy
Crown-rump length (CRL) is the length of the fetus from the top of the head to the rump. In the first two months it grows fast and very predictably, which makes it the best tool for aging an early pregnancy. Reference values for cattle are about 0.9 cm before day 30, 1 cm at day 35 and 2 cm at day 40. Calving dates predicted from CRL before day 55 have landed within about 4.5 days on average.
Got a fetal age from the scan? Put it into the chart tool above as days pregnant, and you’ll get every later milestone on a real date.
Bovine gestation chart FAQ
When can you tell if a cow is pregnant by ultrasound?
From about day 27 to 30 after breeding, when the embryo and its heartbeat can be seen reliably.
When can a vet feel a pregnancy by palpation?
An experienced vet can feel the amniotic vesicle from about day 35 and the membranes from about day 40. It gets much easier after day 60.
How big is a bovine fetus at 3 months?
About the size of a rat. By 5 months it is roughly the size of a large cat.
What is the most accurate way to age a bovine pregnancy?
Crown-rump length on ultrasound in the first two months. Later on, measuring the width of the fetal head is better than judging placentome size.
Last updated October 21, 2026 · Every date on this page comes from the same data the calculators use.