Quick answer: Each month, an ovary releases an egg and the lining of the uterus becomes thick and spongy, with many blood vessels, ready to nourish an embryo. If the egg is not fertilised, this lining is not needed: it slowly breaks down and comes out through the vagina as blood and mucus. This is menstruation. It happens roughly every month and usually lasts for about 2 to 8 days.
In humans, the ovary releases one egg roughly every month. At the same time, the uterus prepares to receive a fertilised egg. If fertilisation happens, the uterus nourishes the developing embryo. If it does not, the uterine lining is shed and the cycle begins again. This repeating cycle is called the menstrual cycle.
In CBSE Class 10 Science, Chapter 7 (How do Organisms Reproduce?), the question "What happens when the egg is not fertilised?" is frequently asked in board exams.
Syllabus note: The NCERT Class 10 textbook explains what happens to the uterine lining when the egg is not fertilised. The hormonal phases of the cycle (FSH, LH, oestrogen, progesterone and the corpus luteum) are covered in Class 12 Biology. They are included below as background, clearly marked as beyond the Class 10 textbook.
What You Will Learn
- What the menstrual cycle is, and the terms menarche and menopause
- What happens to the uterus when the egg is not fertilised (Class 10 answer)
- What happens when the egg is fertilised
- An overview of the phases and hormones of the cycle (beyond the Class 10 textbook)
1. Key Terms
| Term | Meaning |
|---|---|
| Menstrual cycle | The monthly cycle of changes in the ovary and uterus |
| Menstruation | The shedding of the uterine lining as blood and mucus |
| Menarche | The first menstruation, at puberty (usually around 10 to 15 years of age) |
| Menopause | The permanent end of menstrual cycles (usually around 45 to 55 years of age) |
The length of the cycle is often given as about 28 days, but it varies from person to person and from cycle to cycle.
2. What Happens When the Egg Is Not Fertilised? (Class 10)
Common board question: What happens when the egg is not fertilised?
Answer:
- Every month, the ovary releases one egg.
- The uterus prepares itself for a fertilised egg: its lining becomes thick and spongy, with many blood vessels, so that it can nourish an embryo.
- If the egg is not fertilised, this lining is not needed.
- The lining slowly breaks down and comes out through the vagina as blood and mucus.
- This is called menstruation. It happens roughly every month and usually lasts for about 2 to 8 days.
3. What Happens When the Egg Is Fertilised?
- The egg is fertilised by a sperm in the oviduct (fallopian tube), forming a zygote.
- The zygote divides and develops into an embryo, which travels to the uterus.
- The embryo is implanted in the thick lining of the uterus.
- A special tissue called the placenta develops, through which the embryo gets nutrition and oxygen from the mother's blood and passes waste to it.
- The uterine lining is kept, so menstruation does not occur during pregnancy.
4. Phases and Hormones of the Cycle (Beyond the Class 10 Textbook)
This overview uses a typical 28-day cycle; actual days vary.
| Phase | Approximate days | What happens |
|---|---|---|
| Menstrual phase | 1 to 5 | The uterine lining is shed as menstrual flow |
| Follicular phase | 6 to 13 | FSH from the pituitary helps an egg mature in a follicle; the follicle makes oestrogen, which rebuilds the uterine lining |
| Ovulation | Around 14 | A surge of LH from the pituitary causes the follicle to release the egg |
| Luteal phase | 15 to 28 | The empty follicle becomes the corpus luteum, which makes progesterone to maintain the thick uterine lining |
If the egg is not fertilised, the corpus luteum breaks down after about two weeks, progesterone levels fall, and the lining is shed, starting the next cycle.
If the egg is fertilised, hormones from the developing embryo and placenta keep progesterone levels high, so the lining is maintained.
5. Summary Table
| Situation | Uterine lining | Result |
|---|---|---|
| Egg released, not fertilised | Breaks down | Menstruation (blood and mucus) |
| Egg fertilised | Maintained | Implantation, placenta forms, no menstruation |
6. Exam Tips and Common Mistakes
Exam Tip: For "what happens when the egg is not fertilised", mention the thick, spongy lining, that it breaks down, and that it comes out as blood and mucus called menstruation.
Exam Tip: State that menstruation occurs roughly every month and lasts for about 2 to 8 days.
Common Mistake: Saying that fertilisation happens in the uterus or ovary. It happens in the oviduct (fallopian tube).
Common Mistake: Writing that the lining is shed during pregnancy. During pregnancy the lining is maintained to support the embryo.
Frequently Asked Questions
What happens when the egg is not fertilised?
The thick, spongy lining of the uterus is not needed, so it slowly breaks down and comes out through the vagina as blood and mucus. This is called menstruation.
How long does menstruation last?
It usually lasts for about 2 to 8 days and happens roughly every month.
Where does fertilisation take place in humans?
In the oviduct (fallopian tube).
Why does menstruation stop during pregnancy?
The uterine lining is needed to support the developing embryo, so it is maintained instead of being shed.
What are menarche and menopause?
Menarche is the first menstruation at puberty. Menopause is the permanent end of menstrual cycles, usually around 45 to 55 years of age.