How does a baby develop before birth?

1. Before a baby is born, it spends about nine months growing inside its mother, in a stretchy bag of muscle called the womb. What happens in all that time?
2. At the start, the baby is a tiny ball of cells, smaller than a poppy seed. The cells divide again and again. By two months it is the size of a raspberry, with a beating heart.
3. The baby floats in a bag of warm liquid. The liquid cushions bumps, keeps the baby warm and gives it room to wiggle and stretch.
4. The baby does not eat or breathe air yet. An organ called the placenta passes food and oxygen from mom's blood, through the cord, to the baby. Waste travels back the other way.
5. Month by month the baby grows: lime, avocado, banana. Its shape changes too. At first the head is half of its whole length. Later the body and legs catch up.
6. In the last months the baby can hear voices, opens its eyes and puts on weight. After about nine months it is ready to meet its family.

Tap each part of this simple picture to find out what it does for the baby.

• The womb. The womb is a strong, stretchy bag of muscle inside the mother. It grows from about the size of a pear to the size of a watermelon to make room.
• The water cushion. The baby floats in warm liquid inside a thin bag. The liquid works like a cushion and lets the baby move easily.
• The placenta. The placenta grows on the wall of the womb. It passes food and oxygen from mom's blood to the baby and takes waste away.
• The cord. The umbilical cord is a bendy tube that joins the baby's tummy to the placenta. Your belly button is where your cord once was!
• The baby. The baby moves, stretches, sleeps and wakes. From about the middle of the nine months, mom can feel the kicks.
• A tiny heart. The baby's heart starts beating very early, when the baby is smaller than a pea. It beats about twice as fast as a grown-up's heart.

Drag the star along the timeline. Watch the baby's size and shape change, and tap the placenta to send food and oxygen down the cord.

Experiment: As the baby grows, what happens to its head compared with the rest of its body?

1. Drag the star to month 2 and look at the two colored bars beside the baby.
2. Drag slowly to month 5, then on to month 9.
3. Compare the amber head bar with the blue body bar each time.

What we found: At two months the head is about half of the baby's whole length. By nine months it is only about a quarter. The head keeps growing, but the body, arms and legs grow faster and catch up.

Turn on JavaScript to try it yourself.

Each card names a moment. Drag the star to the time it first happens, then press That is when! Watch the picture for clues.

Every person started out smaller than a poppy seed, as a tiny ball of cells. The cells divided again and again, and soon they began to take on different jobs: some became heart, some skin, some brain. All of this happens in the womb, a stretchy bag of muscle inside the mother that grows as the baby grows.

A baby in the womb does not eat with its mouth or breathe air. Everything it needs comes through the placenta, an organ that grows on the wall of the womb just for this job. Food and oxygen pass from the mother's blood into the baby's blood and travel down the cord to the baby's tummy. Waste goes back the other way. Your belly button shows where your cord was joined.

The baby floats in warm liquid that cushions it like a pillow. As the months go by it grows from the size of a raspberry to a lime, a banana and at last about as heavy as a small watermelon. Its shape changes too: early on, the head is half of the whole baby, and later the body and legs catch up.

Our seed comparison is not perfect. A seed carries its own packed lunch and grows alone, but a baby gets a fresh supply from its mother every moment. Babies also grow at their own pace, so sizes are always about, never exactly.

Key words

Womb

The stretchy, muscular bag inside a mother where a baby grows before it is born.

Placenta

An organ that grows on the wall of the womb and passes food and oxygen to the baby and takes waste away.

Umbilical cord

The bendy tube that joins the baby to the placenta.

Amniotic fluid

The warm liquid that surrounds and cushions the baby.

Embryo and fetus

Names for a baby before birth: embryo for about the first two months, fetus after that.

Check your thinking

1. How does a baby get food and oxygen before it is born?

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   From mom's blood, through the placenta and the cord. The placenta passes food and oxygen from mom's blood to the baby's blood, and the cord carries them to the baby.

2. What does the warm liquid around the baby do?

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   It cushions the baby and gives it room to move. The liquid spreads out bumps and lets the baby wiggle, which helps its muscles grow.

3. How does the baby's shape change as it grows?

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   The head becomes a smaller share because the body and legs catch up. The head is about half of the baby at two months and about a quarter at nine months.

4. In this model, when is most of the baby's mass added?

   Show the answer

   In the last three months. A typical fetus weighs about 1 kilogram at 28 weeks and over 3 kilograms at 40 weeks, so more than half of the mass is added near the end.

5. What does your belly button show?

   Show the answer

   Where your cord was joined to you. The umbilical cord joined your tummy to the placenta. The belly button is the mark it left.