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Pregnancy and birth

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  • Additional HL/
  • HL Animal Physiology Activities for Learning/
  • Pregnancy and birth

Students study simply how the hormone HCG stimulates the ovary to secrete progesterone during early pregnancy. Once the embryo is implanted, its placenta facilitates the exchange of materials between the mother and fetus while continuing the production of estrogen and progesterone until positive feedback involving oxytocin leads to birth. Using simple diagrams and IB style questions students make a few notes and test their understanding.

Lesson Description

Guiding Questions

  • How does a newly implanted blastocyst prevent the monthly period of the menstrual cycle?
  • What are the roles of the placenta during a pregnancy?
  • How long does a pregnancy need to be?

Activity 1 - Detecting pregnancy and hCG

This image shows the changes in three hormones during pregnancy.
Study the image then answer the IB style questions on hCG which follow.

This questions below about hCG levels in pregnancy must have been the cause of great excitement and some confusion for future parents who don't understand the role of hCG in early pregnancy. Using the information in the image above answer these questions.

Activity 2 - Pregnancy, the placenta's role and positive feedback in birth

Study this explanation of the role of the placenta during pregnancy and how positive feedback leads to birth. Then answer the questions below. (click the eye icon to view the diagram)

Answer the questions on the Placenta and birth IB style questions worksheet​.

Activity 3 - Gestation period lengths and animal size

The following infographic shows some data about gestation period (pregnancy length) an lifespan and size of different animals. Does there seem to be a correlation between gestation period and lifespan?

GoFigure takes a look at the gestation periods of various animals.
Source:LiveScience

The graph below shows data of adult female mass and pregnancy length of a range of quadrupeds.

Study the graph and answer the following questions

  1. The gestation period of humans is 266 days, according to the 'trend line' on the graph what should the average adult female mass be for humans?

    ..............................................................
  2. Is this figure higher or lower than the average human female mass of approx 60kgm?
    Suggest reasons why?

    .......................................................................

Click the eye icon for some suggested answers:

  1. The gestation period of humans is 266 days, according to the 'trend line' on the graph what should the average adult female mass be for humans?

    ..... 275 kg ...
  2. Is this figure higher or lower than the average human female mass of approx 60kgm?
    Suggest reasons why?

    .The mass is much higher. It seems that humans are pregnant for longer than might be necessary just for the body size of the baby to grow.
    Some possible reasons why could include:
    1. brain development takes longer in humans than other animals
    2. human metabolism is slower than other animals (unlikely)
    3. humans have fewer babies at a time compared to mice or dogs etc ...

Further reading

This is quite a nice simple discussion of possible reasons for human pregnancy being nine months

Some might prefer A scientific paper about gestation length and biological factors

Teacher's notes

In the first two activities students study a simple diagram. This could be done as a whole class using the images on the whiteboard. Students then answer some summary questions about the key details.

In activity one there is an opportunity to illustrate the nature of science, in particular in Biology where data is very variable how it is still possible to draw conclusions and also the importance of large data sets.

For activity two this youtube video gives an excellent explanation of the homones during pregnancy. While it is aimed at medical students and contains a couple of extra hormone details, it is a clear explanation of the roles of estrogen, progesterone, and oxytocin.

The last activity looks at some data about gestation periods and animal size, showing how the growth of mammals occurs at a surprisingly similar rate, presumably because the metabolism of all mammals is so similar.

Model answers can be found here: