It takes nine months for an embryo to grow into a baby inside its mother’s uterus (womb). After birth, the baby keeps growing for another 20 years, changing all the time. Growth is quickest in the first few years of life.
An embryo begins life as one cell. It divides repeatedly to form a ball of cells and starts to change shape. After four weeks it has a brain and a backbone. At six weeks it has limbs and its heart starts beating. At 12 weeks it looks like a miniature baby.
Children change shape as they grow because parts of the body grow at different speeds. The brain grows quickest at first, which is why babies have such large heads. Muscles and bones grow later on. The rate of growth slows down during childhood, but shoots up again at puberty.
The period when a child changes into an adult is called puberty. For girls, puberty usually begins between 10 and 12 years. They grow taller, their breasts develop, and their hips widen. Hair grows under the arms and around the groin. Girls’ ovaries start to release ova each month and they have periods.
Boys go through puberty later than girls—between 12 and 14 years. They shoot up in height, and their shoulders and chests get broader. Hair starts to grow on the face, under the arms, around the groin, and sometimes on the chest. The voice gets deeper, and the testes start to make sperm.
After early adulthood, the body gradually begins to decline. The skin loses its elasticity, muscles get weaker, and internal organs become less efficient. Certain diseases become more common as we get older, including heart disease and cancer. Aging is a very slow process, and most people lead active lives well into their 70s and 80s.
A baby’s brain has a full set of brain cells, but there are few connections between them. The brain grows quickly in the first two years.
The brain is nearly adult size, but continues to change and learn by growing new connections between the brain cells.