Describe changing oestrogen and progesterone sources in pregnancy
Pregnancy source map: ovarian corpus luteum contributes early; the placenta later becomes a major source of oestrogen and progesterone. The transition is gradual, not instant at fertilisation.
Section 16.5 · Sex hormones in humans
Extended only (Supplement); Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
Before you begin
- Recall endocrine glands, blood-borne hormones, ovaries, testes, uterine lining, ovulation and implantation.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which pairing correctly gives early and later sources?
What you will learn
- Separate early ovarian support from later placental production
- Avoid an instant source-switch explanation
Original teaching model.
| Feature | Meaning |
|---|---|
| Early pregnancy | Ovarian corpus luteum |
| Later pregnancy | Placenta becomes a major source |
| Hormones | Oestrogen and progesterone |
| Model limit | No universal switch day or clinical value |
Separate early ovarian support from later placental production
The sites of oestrogen and progesterone production change in their relative importance during pregnancy. The ovarian corpus luteum is important in early pregnancy and continues to support hormone production. The developing placenta later becomes a major source of these hormones. An answer should name both the early ovarian structure and the later placental source instead of choosing only one for the entire pregnancy.
The transition is gradual rather than an instantaneous switch at fertilisation. A newly formed zygote does not already have a mature placenta, and early implantation does not mean the ovarian corpus luteum has immediately stopped functioning. The source diagram therefore separates early and later stages without imposing one exact day as a universal boundary.
Avoid an instant source-switch explanation
Oestrogen and progesterone produced during pregnancy contribute to continued uterine support. Progesterone helps maintain the lining instead of the ordinary cyclic withdrawal followed by menstruation. The placenta has both an exchange role and an endocrine role. Maternal and fetal blood systems normally remaining separate does not prevent a placental tissue from producing hormones.
The syllabus focus is the production sites and the four named hormones in cycle and pregnancy control. Other pregnancy signals and detailed biochemical pathways are outside the explanation required here. The source map gives a stage relationship, not a blood-test interpretation, individual viability prediction or hormone treatment schedule.
Pause and explain
How should the source transition be described?
Worked example
Name the important progesterone sources early and later in pregnancy.
Show the worked solution
- Name early corpus luteum in ovary.
- Name later major placental source.
Answer Early pregnancy: ovarian corpus luteum. Later pregnancy: placenta becomes the major source. The transition is gradual.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Give early and later pregnancy sources for oestrogen and progesterone at this syllabus level.
Give me a hint
Use ovarian corpus luteum and placenta.
Compare my reasoning
- Identify early corpus-luteum contribution.
- Identify later major placental contribution.
The ovarian corpus luteum contributes early; the placenta later becomes a major source of oestrogen and progesterone.
Look for these in your work
- Name both sources
- State the stage relationship
Explain why “fertilisation immediately replaces the corpus luteum with a mature placenta” is inaccurate.
Give me a hint
Separate nuclear fusion from later placental development.
Compare my reasoning
- Recognise a zygote forms at fertilisation.
- Retain early corpus-luteum support and gradual placental development.
Fertilisation forms a zygote, not a mature placenta. The corpus luteum supports early hormone production; placental production becomes important as the placenta develops.
Look for these in your work
- Correct the developmental timing
- Include early support
A source map shows corpus luteum early and placenta later but supplies no blood-test data. What can and cannot be concluded?
Give me a hint
Use source stages without a clinical inference.
Compare my reasoning
- State the changing sources.
- Reject a personal concentration or viability claim.
The map supports the early ovarian and later placental source relationship; it does not measure an individual hormone concentration or establish a pregnancy outcome.
Look for these in your work
- Give the supported relationship
- Name the evidence limit
Common mistakes
- A zygote is not a mature placenta.
- Later placental production does not erase early ovarian support.
What can you explain now?
How should the source transition be described?
Compare with the explanation
Gradual, with changing relative importance
It is not an instant universal-day switch.
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Make it stick
Explain separate early ovarian support from later placental production and avoid an instant source-switch explanation. Follow the event arrows or graph labels before opening the solutions.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.