1Forty weeks
A pregnancy (gestation) lasts about 40 weeks. For the first 8 weeks the developing baby is an embryo; once its main organs have formed, it is called a foetus.
gestationembryofoetus
Lesson 10.3 notes
Pregnancy, the placenta and birth
See how a baby grows for 40 weeks, how the placenta passes it what it needs, how the mother's lifestyle can affect it, and how it is born.
2Inside the uterus
The foetus floats in amniotic fluid, inside a bag called the amniotic sac. The umbilical cord joins it to the placenta, which is attached to the wall of the uterus.
amniotic fluidumbilical cordplacenta
3The placenta
In the placenta, the mother's blood flows very close to the foetus's blood. Oxygen and food diffuse into the foetus's blood; carbon dioxide and waste diffuse back to the mother.
placentadiffusion
4The umbilical cord
The umbilical cord carries the foetus's blood to the placenta and back. In the placenta, the blood picks up oxygen and food and drops off carbon dioxide and waste.
umbilical cord
5A cushion of fluid
The amniotic fluid cushions the foetus, protecting it from bumps and knocks. The strong muscle wall of the uterus protects it too, and the closed cervix holds it in.
amniotic fluid
6The mother's lifestyle
Harmful substances can cross the placenta too. Smoking passes on carbon monoxide, so the foetus gets less oxygen. Alcohol and some drugs can harm how it grows and develops.
placenta
7Birth
After about 40 weeks, the muscles in the uterus wall contract and the cervix widens. The baby is pushed out through the vagina, usually head first. The placenta follows.
contractions
Explore: Placenta lab
Choose what is in the mother's blood, and watch what crosses the placenta to the foetus and what comes back.
Common mistake: The two bloods don't mix
In the placenta, the mother's blood and the baby's blood flow very close together, but a thin barrier keeps them apart. Substances cross it by diffusion.
Remember
- A pregnancy (gestation) lasts about 40 weeks. From about 8 weeks, the embryo is called a foetus.
- The placenta passes oxygen and food to the foetus and takes away carbon dioxide and waste, by diffusion. The umbilical cord joins them.
- Alcohol, carbon monoxide from smoking, and some drugs can cross the placenta too. At birth, the uterus muscles contract to push the baby out.
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