How Can You Tell if a Hippo Is Pregnant?
Researchers use a technique called enzyme immunoassays to monitor hormone levels in our animals. A fecal or urine sample is collected from the animal three times each week. These samples are then assayed to determine how hormone levels change over time.

When trying to determine if a female animal is pregnant, we look at a hormone called progesterone. When a female is not pregnant, levels of this hormone shows very clear patterns during the reproductive cycle of the female. Hormone levels increase and decrease in very regular patterns. However, if a female does get pregnant, levels of this hormone increase and stay at higher levels throughout the pregnancy. You can see this depicted in the graph below for a female hippo.

Figure: Fecal progestagen profiles in a single female hippopotamus
showing ovarian cycles (dashed lines), pregnancy (solid blue line),
and birth (arrow).