It is amazing how fast the little cells start to divide when a sperm fertilizes an egg. It is like watching a miracle in progress. Knowing what is going on inside your growing belly is one of the most exciting parts of pregnancy. Below you will learn how your baby develops during its nine months inside you.

As soon as a sperm fertilizes the egg that your body sent down one of your fallopian tubes, cellular development begins. This new cluster of cells, smaller than a grain of salt, is called an embryo and has inherited 46 chromosomes—half from you and half from the birth father.

The first thing the embryo does is claim a space on your uterus wall, where it embeds itself between days five and nine. As soon as it has attached itself, it starts letting your hormones know so that your body does not have another period. After 17 days, the embryo develops blood cells, and within 19 days it has eyes. Some of the first things to develop are the brain and spinal cord, which make up the nervous system. Your baby has the ability to feel and experience different sensations within its first three weeks. Within 21 to 25 days, a miniature heart is beating.

After just a month of existence, your embryo has a head with ears and an odd-shaped trunk with little buds that will blossom into arms and legs. The embryo measures about a half-inch long and weighs less than an ounce. A sack surrounds it, filled with a special amniotic fluid that nourishes the embryo. The placenta does not let the mother’s blood in, because your baby has its own blood now. But it does let food and oxygen in from your body through the umbilical cord.

About the time you miss your second period, the embryo has a complete skeleton with reflexes and brain waves. A little liver has developed that removes impurities from the blood, and the brain controls muscle movement.

At eight weeks, your embryo is officially a fetus. The fetus has a heart, stomach, liver, and brain and most of the other important systems we take for granted every day. It has ears, ankles, wrists, fingers and toes. The eyelids are sealed shut to protect its highly sensitive eyes. Your fetus is only one inch long and weighs little more than an ounce.

At ten weeks, you will most likely know for sure that you are pregnant. At ten weeks, the baby can squint, swallow, and stick out their tongue. They even have a full set of fingerprints.

By the time the baby is three months old, it will be kicking and thrashing, sucking its thumb, and exercising its lungs by inhaling and exhaling. Soft little nails cover the fingertips and toes, and 20 buds appear that will later become teeth. At this point the baby has hair and kidneys, as well as a bladder to urinate with. The baby sleeps, wakes, and exercises. At four inches long, the detail is similar to a fully developed body.

At four months, the baby has developed the organs that make it a male or female, and you can feel her kick. The baby has grown to six inches now, weighs about five ounces and can pass urine. In fact, the baby circulates about 300 quarts of amniotic fluid a day.

At five months, the baby is growing fast. The vocal cords work, and the baby reacts to loud sounds. It also weighs almost a pound and is eight to twelve inches long.

At six months, or about 26 weeks, the baby is viable, meaning that it might be able to live outside the womb with the help of emergency neonatal care.

At seven months, the baby measures 15 inches long, weighs about three pounds and can recognize your voice.

At eight months, the baby is too big to move around much, but hits and kicks often. The little body is building up antibodies to protect it from sickness outside the womb. At 18 inches long, it now weighs about five pounds.

At nine months, the baby is “full term” and ready to be born. The baby weighs between six and nine pounds, and his or her heart pumps 300 gallons of blood per day. The head bones are soft and flexible to make birth easier. That is why most babies have that “conehead” look when they are first born.