Animal gestation and incubation calculator

How animal gestation differs from incubation

Mammals carry developing young inside the body, so their gestation period is counted from mating or conception to birth. Birds develop in eggs outside the parent, so the calculator treats the bird entries as incubation periods from laying to hatch. The date math is the same, but the biology is different: a cow, dog, cat, gorilla, whale, or elephant is pregnant; a chicken, duck, ostrich, swan, eagle, or penguin egg incubates.

Elephant pregnancies are famous because they run about 22 months. A large calf needs time to develop the brain, body size, coordination, and trunk control needed for a herd-moving newborn. Smithsonian material lists Asian elephant gestation near 21.5 to 22 months, which is why this calculator uses 660 days for the typical estimate.

Blue whales are included because the largest animal on Earth is a sea mammal with a roughly year-long pregnancy. The table also includes short marsupial and rodent periods: opossums can give birth in under two weeks, while mice and hamsters are measured in days rather than months.

Sources include Smithsonian National Zoo, San Diego Zoo Wildlife Alliance, veterinary references including Merck and AVMA Journals, NOAA, Cornell Lab of Ornithology, and per-row citations in the calculator.