BiologyICSE Class 10 · Reproductive System

Fertilisation, Implantation, Placenta and Foetal Membranes (ICSE Class 10)

Updated 16 October 2026 · Fact-checked

A sperm fuses with the ovum in the oviduct to form a zygote (fertilisation). The dividing embryo embeds in the uterine wall (implantation) and forms the placenta, through which it receives food and oxygen and passes out carbon dioxide and urea. The amnion encloses it in amniotic fluid, which protects it until birth.

Understand Fertilisation, Implantation, Placenta and Foetal Membranes

Fertilisation. Sperms deposited in the vagina swim through the cervix and uterus into the oviducts. If an ovum is present, one sperm enters it; the sperm nucleus fuses with the ovum nucleus. This is fertilisation, and it forms a diploid zygote (23 chromosomes from each parent, 46 in all). Fertilisation normally takes place in the oviduct (Fallopian tube).

From zygote to implantation. The zygote divides repeatedly by mitosis as it moves down the oviduct, forming a ball of cells (the early embryo). It reaches the uterus a few days later and, about a week after fertilisation, embeds itself in the thick uterine lining (endometrium). This is implantation; pregnancy has begun.

The placenta. After implantation, finger-like villi grow from the embryo's outer membrane (the chorion) into the uterine wall. The villi and the uterine tissue together form the placenta, a disc-shaped organ attached to the uterine wall. The villi are surrounded by spaces filled with the mother's blood, which gives a large surface area for exchange.

The mother's blood and the foetus's blood do not mix; substances pass across thin membranes.
Nutrition: glucose, amino acids, fats, vitamins, minerals and water pass from mother to foetus.
Respiration: oxygen passes from mother to foetus; carbon dioxide passes back.
Excretion: urea and other wastes pass from foetus to mother, whose kidneys remove them.
Protection: some antibodies pass from mother to foetus; many germs are kept out, though some viruses, alcohol, nicotine and certain drugs can cross.
Endocrine function: the placenta secretes hormones, including progesterone and oestrogen, that maintain pregnancy.

Why the bloods are kept separate. The mother and the foetus may have different blood groups, which could clump if mixed, and the mother's higher blood pressure could damage the delicate blood vessels of the foetus.

The umbilical cord. It joins the foetus to the placenta. Its two umbilical arteries carry blood with carbon dioxide and wastes from the foetus to the placenta; its umbilical vein carries blood rich in oxygen and food to the foetus. After birth the cord is cut; the scar is the navel.

Foetal membranes.

Amnion: the inner membrane, which encloses the foetus in amniotic fluid.
Chorion: the outer membrane; its villi help to form the placenta.

Amniotic fluid: protects the foetus from mechanical shocks, keeps the temperature and pressure around it even, prevents drying and sticking to the amnion, and lets it move freely.

Gestation and parturition.

Gestation: the period from fertilisation to birth, about 280 days (nine months) in humans.
Parturition: birth of the baby, by strong rhythmic contractions of the uterine muscles, stimulated by the hormone oxytocin. The cervix widens (dilates) to let the baby pass. After the baby, the placenta is also expelled as the 'afterbirth'.

Fertilisation, Implantation, Placenta and Foetal Membranes: key facts

Sequence: Ovulation (ovary)
Fertilisation (oviduct)
Zygote divides: ball of cells
Implantation (uterine wall, about a week later)
Placenta forms
Gestation about 280 days
Parturition; Fertilisation in the oviduct; implantation in the uterus.
Placenta: Mother to foetus food, oxygen, water, antibodies
Foetus to mother carbon dioxide, urea
Hormones progesterone, oestrogen; The two bloods never mix.
Umbilical cord: Umbilical vein: oxygen and food to the foetus | Umbilical arteries: carbon dioxide and wastes to the placenta; Two arteries, one vein.
Membranes and fluid: Amnion inner, holds amniotic fluid
Chorion outer, forms villi of placenta
Amniotic fluid shock absorber, even temperature, free movement; Fluid protects; placenta feeds.

Mother to foetus	food, oxygen, water, antibodies
Foetus to mother	carbon dioxide, urea
Hormones	progesterone, oestrogen

Amnion	inner, holds amniotic fluid
Chorion	outer, forms villi of placenta
Amniotic fluid	shock absorber, even temperature, free movement

How to answer Fertilisation, Implantation, Placenta and Foetal Membranes questions

Most questions ask where something happens, what passes which way, or what a structure does.

1Place each event: fertilisation in the oviduct, implantation in the uterine wall, exchange in the placenta.
2For exchange questions, state the direction: mother to foetus or foetus to mother.
3Keep functions apart: the placenta exchanges and secretes hormones; the cord carries; the amniotic fluid cushions.
4When asked 'why', link the structure to its feature: villi give a large surface; separate bloods prevent clumping and damage.
5Give functions when functions are asked, not locations.

Fertilisation, Implantation, Placenta and Foetal Membranes in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Your navel

Suppose a young child asks, 'Why does everyone have a belly button?'

Explain using the umbilical cord.

Try it first, then show the solutionSolution

Before birth, the foetus was joined to the placenta by the umbilical cord.
Through it the foetus received oxygen and food and sent back carbon dioxide and wastes.
After birth the baby breathes and feeds itself, so the cord is clamped and cut; the stump dries and falls off, leaving the navel.

Answer: The navel is the scar where the umbilical cord was attached.

Memory hook: The navel is the old supply line's scar.

Protection passed on

Imagine a pregnant woman is given a vaccine by her doctor, who says it will also protect her newborn baby for the first few weeks.

How can a vaccine given to the mother protect the baby?

Try it first, then show the solutionSolution

The vaccine makes the mother's body produce antibodies.
Some of these antibodies pass across the placenta into the foetus's blood.
The baby is born with these antibodies, which protect it until it can make its own.

Answer: Antibodies cross the placenta from mother to foetus.

Memory hook: The placenta passes on protection as well as food.

Tricks to remember Fertilisation, Implantation, Placenta and Foetal Membranes

Short memory aids. Say each one aloud once, then use it on a question the same day.

Oviduct to fertilise, uterus to implant
'O' for oviduct comes before 'U' for uterus: fertilise first, implant later.
Use it when: Sites of fertilisation and implantation.
Vein brings, arteries take
In the cord the vein brings oxygen and food to the baby; the arteries take wastes away (opposite to the adult rule).
Use it when: Umbilical vessels.
A-in, C-out
Amnion inside, chorion outside.
Use it when: Foetal membranes.

Fertilisation, Implantation, Placenta and Foetal Membranes: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Which way across the placenta? · 1 mark · MCQBoard level

Which of these normally passes from the foetus to the mother across the placenta?
(a) glucose
(b) oxygen
(c) urea
(d) amino acids

Try it first, then show the solutionSolution

Wastes such as urea go from foetus to mother; the others go from mother to foetus.

Answer: (c) urea

Example 2: Naming the outer membrane · 2 marks · Very short answerBoard level

Name: (a) the outer foetal membrane, part of which helps to form the placenta; (b) the finger-like projections from it that grow into the uterine wall.

Try it first, then show the solutionSolution

(a) Chorion. (1)
(b) Villi (chorionic villi). (1)

Answer: (a) Chorion (b) Chorionic villi

Example 3: Two bloods, kept apart · 2 marks · Short answerHarder board level

In the placenta the mother's blood and the foetus's blood come very close but do not mix. Give two reasons why it is important that they do not mix.

Try it first, then show the solutionSolution

The mother and foetus may have different blood groups; mixing could cause clumping (agglutination). (1)
The mother's blood pressure is much higher and could damage the delicate foetal blood vessels. (1)

Answer: Different blood groups could clump; mother's higher pressure could damage foetal vessels.

Example 4: Built for exchange · 2 marks · Short answerHarder board level

Explain how the structure of the placenta makes it efficient at exchanging materials between mother and foetus.

Try it first, then show the solutionSolution

Numerous villi project into spaces filled with the mother's blood, giving a very large surface area. (1)
The membranes separating the two bloods are very thin, so substances such as oxygen and glucose diffuse across quickly. (1)

Answer: Many villi give a large surface; thin membranes allow quick diffusion.

Example 5: From vagina to uterine wall · 3 marks · Short answerHarder board level

Trace the route taken by sperms from the vagina to the site of fertilisation. Then describe what happens to the fertilised egg until it implants.

Try it first, then show the solutionSolution

Sperms swim from the vagina through the cervix and the uterus into the oviduct, where they meet the ovum. (1)
One sperm fuses with the ovum to form a zygote; the zygote divides repeatedly by mitosis, forming a ball of cells, while moving down the oviduct. (1)
About a week after fertilisation it embeds in the endometrium of the uterus: implantation. (1)

Answer: Vagina → cervix → uterus → oviduct; zygote divides into a ball of cells and implants in the uterine wall.

Example 6: Case study: an embryo in the wrong place · 4 marks · Case-basedHOTS / competency

Suppose, in rare cases, a fertilised egg implants in the wall of the oviduct instead of the uterus. Doctors say such a pregnancy cannot continue normally.
(a) Where should implantation take place, and what is that layer called?
(b) Give two reasons why the embryo cannot develop normally in the oviduct.
(c) Which event, the movement of the zygote or fertilisation, has failed here?

Try it first, then show the solutionSolution

(a) In the uterus, in its thick lining, the endometrium. (1)
(b) The thin oviduct wall cannot form a proper placenta with a rich blood supply to nourish the embryo. (1)
The narrow oviduct cannot stretch like the muscular uterus to hold a growing foetus; it may burst. (1)
(c) Fertilisation succeeded; the zygote failed to move down to the uterus in time. (1)

Answer: (a) Uterus (endometrium) (b) No proper placenta; cannot stretch (c) Movement of the zygote failed

Where students lose marks in Fertilisation, Implantation, Placenta and Foetal Membranes

The slips examiners see most often, and what to write instead.

Mistake: Amniotic fluid confused with the placenta
CISCE examiners' comments, 2025 Q7(iii): asked for functions of amniotic fluid, candidates confused it with the placenta and wrote 'nutrient supply'.
Wrong working
Amniotic fluid supplies food to the foetus.
Right working
Amniotic fluid protects from shocks and keeps the temperature even; the placenta supplies food.
Do this: Fluid cushions; placenta feeds.
Usually costs: 1 mark.
Mistake: Uterus confused with ureter; locations given for functions
CISCE examiners' comments, 2025 Q5(iii): differentiating the functions of urethra and uterus, candidates interchanged functions, wrote locations, or confused uterus with ureter.
Wrong working
Uterus: lies behind the bladder.
Right working
Uterus: site of implantation; houses and nourishes the developing foetus; contracts at birth.
Do this: When a function is asked, give what it does.
Usually costs: 1 mark.
Mistake: Implantation placed in the oviduct
Mixing up the two sites gives a wrong answer for both terms.
Wrong working
Implantation takes place in the oviduct.
Right working
Fertilisation in the oviduct; implantation in the uterine wall.
Do this: Fertilise first (oviduct), implant later (uterus).
Usually costs: 1 mark.

Exam tips

Write the direction of exchange for each substance across the placenta.
Mention that the bloods do not mix whenever you describe the placenta.
Use 'endometrium' or 'uterine wall' for implantation and 'oviduct' for fertilisation.
Keep the functions of the placenta, umbilical cord and amniotic fluid separate.

Practice questions on Fertilisation, Implantation, Placenta and Foetal Membranes

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The blood vessel in the umbilical cord that carries oxygen-rich blood to the foetus is the:
(a) umbilical artery
(b) umbilical vein
(c) pulmonary vein
(d) aorta
Hint
Vein brings.
Answer
(b) umbilical vein
Q2 · 2 marks · Very short answerBoard level
Define fertilisation. How many chromosomes does a human zygote have?
Hint
23 + 23.
Answer
Fertilisation is the fusion of the nucleus of a sperm with the nucleus of an ovum to form a zygote. The zygote has 46 chromosomes.
Q3 · 1 mark · Very short answerBoard level
Give one difference between the amnion and the chorion.
Hint
Inside or outside?
Answer
The amnion is the inner membrane enclosing the amniotic fluid; the chorion is the outer membrane whose villi help form the placenta.
Q4 · 2 marks · Short answerHarder board level
In an adult, arteries carry oxygen-rich blood. Why, then, do the umbilical arteries carry blood that is poor in oxygen?
Hint
Arteries are named by direction, away from the heart.
Answer
Arteries carry blood away from the heart. The umbilical arteries carry blood from the foetus's heart to the placenta; this blood has already given up its oxygen to the foetus's tissues and carries carbon dioxide and wastes, so it is poor in oxygen.
Q5 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The foetus can survive in the uterus without breathing air.
Reason (R): The foetus receives oxygen from the mother's blood across the placenta.
(a) Both (A) and (R) are true, and (R) is the correct explanation for (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation for (A).
(c) (A) is true and (R) is false.
(d) (A) is false and (R) is true.
Hint
Respiratory role of the placenta.
Answer
(a) Both are true, and oxygen supplied through the placenta is why the foetus does not need to breathe.
Q6 · 2 marks · Short answerHarder board level
A pregnant woman is advised to eat food rich in iron and protein. Explain the advice, using the role of the placenta.
Hint
The foetus builds blood and body from the mother's supply.
Answer
The foetus gets all its raw materials from the mother's blood through the placenta. It needs iron to make haemoglobin and protein (amino acids) to build its body, so the mother's diet must supply extra.
Q7 · 2 marks · Short answerHOTS / competency
Late in pregnancy, part of the placenta separates from the uterine wall. Predict two effects on the foetus and explain.
Hint
Less surface in contact with the mother's blood.
Answer
Less exchange surface remains, so the foetus gets less oxygen and food; carbon dioxide and urea build up in its blood. Its growth and health would suffer.
Q8 · 2 marks · Short answerHOTS / competency
Oxygen moves from the mother's blood to the foetus's blood in the placenta, though the two never mix. Explain how, and why it moves in that direction.
Hint
Diffusion down a concentration gradient.
Answer
Oxygen diffuses across the thin membranes of the villi. The mother's blood in the placenta has more oxygen than the foetal blood arriving there, which has given up oxygen to the foetus's tissues, so oxygen moves from mother to foetus.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Hormone that brings on the uterine contractions of birth?Show answer
Oxytocin.
2.Where does the zygote begin to divide?Show answer
In the oviduct, as it moves towards the uterus.
3.Two substances passing from foetus to mother?Show answer
Carbon dioxide and urea.
4.What is the afterbirth?Show answer
The placenta (with the membranes), expelled after the baby is born.
5.Blood vessels in the umbilical cord?Show answer
Two umbilical arteries and one umbilical vein.

Fertilisation, Implantation, Placenta and Foetal Membranes: frequently asked questions

Is the placenta part of the mother or of the baby?

Both. It is formed by the chorionic villi of the embryo together with the uterine tissue of the mother, so it links the two while keeping their blood separate.

What is the difference between fertilisation and implantation?

Fertilisation is the fusion of sperm and ovum nuclei, in the oviduct. Implantation is the embedding of the dividing embryo in the uterine wall, about a week later.

What are the foetal membranes?

The amnion (inner), which encloses the foetus in amniotic fluid, and the chorion (outer), whose villi help to form the placenta.

Does cutting the umbilical cord hurt the baby?

No. The cord has no nerves. After birth it is clamped and cut; the stump dries and falls off within a week or two, leaving the navel.