Human conception to birth date

The length of the average pregnancy is approximately 266 days. As the point of ovulation is usually not known, an estimate of the birth date is made using the day of the last menstrual period. This is usually 14 days before ovulation, so 14 days are added to the figure of 266, making a total of 280 days, or 40 weeks. Only about 5% of women actually give birth on the date they are due.

Giraffes

Calves can be 6 feet tall and weigh 250 pounds when born. After only about an hour, the baby giraffe, encouraged by Mom, takes its first steps. They can’t run fast, and a mother giraffe is very protective. Always on guard, she will sleep for only about 30 minutes a day, a few minutes at a time. Giraffe mothers are pregnant for about 15 months.

Elephants

Their pregnancies last 22 months, the longest for any mammal. Calves nurse for four to six years. Females will stay with their herd for the rest of their lives, while the males will eventually go off on their own. All the females in a herd will help care for the calves, which can weigh about 200 pounds at birth. That's typically about 1/45 of what their mom weighs.

Koalas

A koala is the size of a jellybean at birth. It will spend the first six months of life tucked inside Mom’s pouch. Baby koalas, called joeys, have to develop the ability to digest the one thing they will eat: highly toxic eucalyptus leaves. When they’re born, they don’t have the necessary digestive tolerance. Mom helps them develop it by feeding them her feces.

Orangutans

The nurturing period between mother and baby is one of the longest in nature. Orangutans spend seven to nine years with their moms learning everything they need to know, like how to find food and build nests. Even after orangutans leave the nest, sometimes they will still visit their mom in the forest until they’re about 15 or 16. A mother gives birth every six to eight years. They are pregnant for 8 1/2 months. Usually a single infant is born.

Breeding like ...

A European rabbit can have more than 360 offspring in its lifetime, but only 15% will make it through the first year.

Polar bears

A pregnant polar bear gains about 200 pounds and then digs out a den in a deep snowbank in which to give birth. She will stay tucked away for three months with her newborn cubs until springtime. For the next 2 1/2 years, the cubs will stay close to Mom as she teaches them all the skills they need to live alone. Mothers are pregnant about 195 to 265 days. There are typically two cubs per litter.

Other mammals’ gestation, newborn weight and weight as a fraction of mother’s:

Bengal tiger
103 days
2.2 pounds, 1/120

Giant panda
95-160 days
3.2 ounces, 1/900 (smallest of placental mammals)

Killer whale
457-548 days
265-353 pounds, 1/50