Pregnancy

Pregnancy is the time when a baby grows inside its mother's body. In humans, pregnancy lasts about 40 weeks, or just over nine months. The baby grows inside a special organ called the uterus, which is shaped a bit like an upside-down pear. Almost every mammal goes through pregnancy, but the length is different for each species.

Pregnancy starts when a sperm cell from a father joins with an egg cell from a mother. This is called fertilization. The joined cell holds DNA from both parents. That DNA is the instruction book for everything about the new baby, from eye color to height. Within a few days, the single cell splits into two, then four, then many more.

Doctors usually divide pregnancy into three parts called trimesters. Each one lasts about three months.

In the first trimester, the baby is called an embryo. It is tiny, smaller than a grape at the end of three months. But its heart has already started beating, and the beginnings of its brain, eyes, and tiny arms and legs are forming. The mother often feels tired or sick during this time.

In the second trimester, the baby is now called a fetus. It grows quickly. By month five, the mother can usually feel the baby moving and kicking. The fetus can hear sounds from outside the body, including its mother's voice and music.

In the third trimester, the fetus puts on weight and gets ready to be born. By the end, a typical baby weighs around seven pounds, about the same as a small bowling ball. The baby's lungs are the last big system to finish developing.

The fetus does not eat or breathe the way we do. It gets food and oxygen through a special organ called the placenta, which connects to the mother's body. A long tube called the umbilical cord links the placenta to the baby's belly. After birth, the spot where this cord was attached becomes the belly button.

Sometimes a mother is pregnant with more than one baby at the same time. Twins happen in about one out of every 60 human births. Triplets and quadruplets are much rarer.

Pregnancy ends with birth, when the baby leaves the uterus and enters the world. From that very first cell to a crying newborn, the whole process takes only about 40 weeks.