The earlier a cow's pregnancy is detected, the better it is for both the cow and the unborn calf. If pregnancy is detected early, the cow is more likely to receiving extra medical attention and feed, along with reducing the likelihood that the cow will be intentionally re-bred. According to University of Florida authors Frederico Moreira and P.J. Hansen, re-insemination is a common cause of miscarriage in newly pregnant cows.
It can be difficult or impossible to diagnose a cow's pregnancy through outward physical symptoms, so a variety of medical tests can be used to indicate pregnancy starting shortly after insemination.
Veterinarians or experienced cattlemen can diagnose pregnancy in a cow by detecting signs of estrus, measuring plasma progesterone levels in milk, ultrasonography and traditional rectal palpation.
According to Moreira and Hansen, cows typically show estrus, or signs of heat, approximately every 21 days. After a cow is inseminated, either naturally or by artificial insemination, the animal's corpus luteum will not regress.
The corpus luteum, or yellow body, is a structure that develops from ovarian follicles during ovulation. Moreira and Hanson say that in the event of pregnancy, "The corpus luteum will not regress, progesterone concentrations will remain high, and the cow will not return to estrus."
If the cow is not observed to enter back into estrus approximately 21 days after end of the previous cycle, it is highly possible the cow is pregnant. Visual signs to detect estrus include tail chalking, mucus production from the vagina, swollen vulva and a willingness to be mounted.
Use of Milk or Plasma Progesterone
Moreira and Hansen write that it is "possible to diagnose cows for pregnancy based on milk or plasma progesterone concentrations at about 21 days after insemination." They recommend collecting milk samples a minimum of two times after insemination and testing the samples for progesterone concentration. Cows with elevated progesterone are probably pregnant.
During ultrasonography, an ultrasound probe is inserted through the rectum and positioned above the uterus, generating pulses of ultrasound that produce an electrical signal. According to Moreira and Hansen, the signal is then processed using a scan converter and displayed on a monitor. The embryo of the unborn calf will be visible on the screen. Ultrasonography diagnoses are highly reliable.
Rectal palpation of the uterus and its contents is the most common form of detecting pregnancy in cows, and it can be conducted accurately as early as 35 days into the pregnancy. According to Moreira and Hansen, the examiner conducting the rectal exam will feel for uterine asymmetry, or slippage of the fetal membrane along the uterus. As the pregnancy progresses, the fetus will grow too large to palpate in its entirety, but parts of the fetus will be felt.
Late Pregnancy Indicators
Once the pregnancy continues into its later stages, the cow's abdomen will become larger and veterinarians may be able to detect a heartbeat in the fetus using ultrasound technology.