BiologyCBSE Class 12 · Human Reproduction

Fertilisation, Cleavage and Implantation

Updated 15 October 2026 · Fact-checked

Fertilisation is the fusion of a sperm with the ovum at the ampullary region of the fallopian tube. Sperm contact changes the zona pellucida to block other sperms, and meiosis II of the oocyte is completed. The diploid zygote cleaves into a morula, then a blastocyst, whose trophoblast attaches to the endometrium: implantation, which leads to pregnancy.

Understand Fertilisation, Cleavage and Implantation

Getting sperm and ovum together. During copulation, semen is released into the vagina (insemination). Motile sperms swim through the cervix and uterus into the fallopian tube, to the ampullary region (ampulla) of the fallopian tube. (Older editions called the site the ampullary-isthmic junction; the current NCERT text says ampullary region.) The ovum released at ovulation is carried to the same place. Fertilisation can happen only if the ovum and sperms reach the ampullary region at the same time, which is why not every copulation leads to pregnancy.

Fertilisation step by step.
1. A sperm comes into contact with the zona pellucida of the ovum.
2. This induces changes in the membrane that block the entry of additional sperms (block to polyspermy), so only one sperm fertilises the ovum.
3. Acrosomal secretions help the sperm pass through the zona pellucida and plasma membrane into the cytoplasm.
4. Sperm entry induces the secondary oocyte to complete meiosis II, an unequal division giving the second polar body and a haploid ovum (ootid).
5. The haploid nuclei of sperm and ovum fuse to form a diploid zygote.

Sex is decided at fertilisation. Women are XX and men XY. All ova carry an X; half the sperms carry X and half carry Y. X sperm + X ovum → XX zygote (female); Y sperm + X ovum → XY zygote (male). So, scientifically, the sex of the baby is determined by the father, not the mother, and the chance of a boy or a girl is 1/2 at each birth.

Cleavage. As the zygote moves through the isthmus towards the uterus, it divides by mitosis (cleavage) into 2, 4, 8, 16 daughter cells called blastomeres. The embryo with 8 to 16 blastomeres is the morula. The morula keeps dividing and becomes the blastocyst as it moves into the uterus.

Blastocyst. Its blastomeres are arranged into:

an outer layer, the trophoblast, which attaches to the endometrium (and later helps form the placenta);
an inner cell mass attached to the trophoblast, which differentiates into the embryo. The inner cell mass contains stem cells, which can give rise to all tissues and organs.

Implantation. The trophoblast attaches to the endometrium; the uterine cells then divide rapidly and cover the blastocyst, so it becomes embedded in the endometrium. This is implantation, and it leads to pregnancy. After implantation, finger-like chorionic villi of the trophoblast interlock with uterine tissue to form the placenta (elementary idea only).

Flow chart (place and event):
ampullary region: fertilisation → zygote (2n) → isthmus: cleavage → 2 → 4 → 8 → 16 blastomeres (morula) → uterus: blastocyst (trophoblast + inner cell mass) → endometrium: implantation → pregnancy.

Twins (for HOTS). If the blastomeres of one zygote separate early and each develops, identical (monozygotic) twins of the same sex and genes result. If two ova are released and fertilised by two different sperms, fraternal (dizygotic) twins result; they may differ in sex.

Draw and label 1: transport of ovum, fertilisation and passage of the embryo. Checklist: ovary beside a curved fallopian tube opening into the uterus; show along the tube in order: ovum released, fertilisation at the ampullary region, zygote, 2-cell and 4-cell stages, morula, blastocyst entering the uterus, implanted blastocyst in the endometrium. Labels: ovary, fimbriae, ampulla, isthmus, fertilisation, zygote, morula, blastocyst, implantation, endometrium.

Draw and label 2: blastocyst. Checklist: hollow ball with a single-layered wall and a clump of cells inside on one side. Labels: trophoblast, inner cell mass, blastocoel (cavity).

Fertilisation, Cleavage and Implantation: key facts

Fertilisation: $sperm (n) + ovum (n) \to zygote (2 n)$; At the ampullary region of the fallopian tube.
Sex of the child: $X_{sperm} + X \to X X (girl); Y_{sperm} + X \to X Y (boy)$; Half the sperms carry X and half Y: P(boy) = P(girl) = 1/2.
Cleavage: $1 \to 2 \to 4 \to 8 \to 16 blastomeres; 2^{k} cells after k divisions$; 8-16 blastomeres = morula.
Blastocyst layers: $trophoblast (attaches, forms placenta part) + inner cell mass (embryo)$; Inner cell mass contains stem cells.
Sequence of events: $insemination \to fertilisation \to cleavage \to morula \to blastocyst \to implantation$; Implantation leads to pregnancy.

How to answer Fertilisation, Cleavage and Implantation questions

Questions ask you to trace events, give a site, explain a block, or work out sex or twinning outcomes.

1For 'trace the events', write them in order with the site of each: fertilisation (ampullary region), cleavage (isthmus), blastocyst (uterus), implantation (endometrium).
2For the block to polyspermy, name the layer (zona pellucida) and say sperm contact induces membrane changes that block other sperms.
3Always mention that sperm entry completes meiosis II of the secondary oocyte and gives a second polar body.
4For sex determination, draw a two-row cross: father's sperms X and Y; mother's ova X only; give the 1:1 outcome.
5For probability over several children, multiply 1/2 for each independent birth.
6For blastocyst questions, name both parts and the fate of each; add that inner cell mass contains stem cells.

Fertilisation, Cleavage and Implantation 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.

Sex determination by the father and independent births

Suppose a couple already has two daughters and a relative blames the mother, insisting that their third child 'must be a boy this time'.

Who determines the sex of the child, and what is the probability that the third child will be a boy? What was the probability, before any births, that all three children would be girls?

Try it first, then show the solutionSolution

The mother's ova all carry X; the father's sperms carry X or Y in equal numbers, so the sperm decides: XX girl or XY boy. The mother cannot be blamed.
Each birth is independent: P(boy) for the third child = 1/2, regardless of the first two.
P(three girls) before any births = 1/2 × 1/2 × 1/2 = 1/8.

Answer: The father's sperm decides; P(third child is a boy) = 1/2; P(three girls) = 1/8.

Memory hook: The egg always says X; the sperm casts the deciding vote.

Tricks to remember Fertilisation, Cleavage and Implantation

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

Meet in the ampulla
The ampulla is the meeting room: sperm and egg must arrive there together.
Use it when: Site of fertilisation.
Morula = mulberry
Morula looks like a mulberry: a solid ball of 8-16 cells.
Use it when: Distinguishing morula from blastocyst (hollow).
Troph = Touch, Inner = Infant
Trophoblast touches the uterus; inner cell mass becomes the infant (embryo).
Use it when: Fates of the blastocyst parts.

Fertilisation, Cleavage and Implantation: solved examples

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

Example 1: Block to polyspermy · 2 marks · Very short answerBoard level

A man ejaculates about 200-300 million sperms, yet an ovum is fertilised by only one. How does the ovum block entry of additional sperms? Why does every copulation not lead to fertilisation?

Try it first, then show the solutionSolution

When a sperm contacts the zona pellucida, it induces changes in the membrane that block the entry of additional sperms (1 mark).
Fertilisation happens only if the ovum and sperms reach the ampullary region at the same time; otherwise no fertilisation occurs (1 mark).

Answer: Zona pellucida changes after first sperm contact; ovum and sperms must meet at the ampullary region simultaneously.

Example 2: Morula to pregnancy · 3 marks · Short answerBoard level

Explain the phases in embryonic development from the morula stage to the establishment of pregnancy.

Try it first, then show the solutionSolution

The morula (8-16 blastomeres) keeps dividing and becomes a blastocyst as it moves into the uterus (1 mark).
The blastocyst has an outer trophoblast and an inner cell mass; the inner cell mass will differentiate into the embryo (1 mark).
The trophoblast attaches to the endometrium, uterine cells divide rapidly and cover the blastocyst, embedding it: implantation, which leads to pregnancy (1 mark).

Answer: Morula → blastocyst (trophoblast + inner cell mass) → trophoblast attaches → embedded in endometrium (implantation) → pregnancy.

Example 3: Fertilisation to implantation · 5 marks · Long answerHarder board level

Trace the events from fertilisation to implantation in a human female, naming the site of fertilisation and the role of the acrosome.

Try it first, then show the solutionSolution

Site: ampullary region of the fallopian tube, reached by both ovum and sperms (1 mark).
Sperm contacts the zona pellucida, which changes to block other sperms; acrosomal secretions help the sperm through the zona pellucida and plasma membrane (1 mark).
Sperm entry completes meiosis II: ovum (ootid) + second polar body; sperm and ovum nuclei fuse into a diploid zygote (1 mark).
Zygote cleaves while moving through the isthmus: 2, 4, 8, 16 blastomeres; 8-16 blastomeres = morula; morula becomes blastocyst in the uterus (1 mark).
Trophoblast attaches to the endometrium and the blastocyst is embedded: implantation; inner cell mass forms the embryo (1 mark).

Answer: Ampullary region; zona block and acrosome; meiosis II; zygote; cleavage → morula → blastocyst; implantation.

Example 4: Ploidy along the tube · 2 marks · Very short answerHarder board level

A figure shows the ovum, the zygote, a 4-cell stage and a morula along the fallopian tube. Give the ploidy of the cells at each stage and name the division that produces blastomeres.

Try it first, then show the solutionSolution

Ovum (secondary oocyte at release, ootid after meiosis II): haploid; zygote: diploid (1 mark).
4-cell stage and morula: diploid, because cleavage is mitosis (1 mark).

Answer: Ovum n; zygote, 4-cell stage and morula 2n; cleavage is mitotic.

Example 5: Identical or fraternal? · 3 marks · Short answerHOTS / competency

Suppose twin babies are born, one boy and one girl. (a) Are they identical or fraternal? Explain using the events of fertilisation. (b) How would identical twins arise? (c) Could identical twins ever be of different sexes? Why?

Try it first, then show the solutionSolution

(a) Fraternal (dizygotic): two ova were released and fertilised by two different sperms, one X-bearing (girl) and one Y-bearing (boy) (1 mark).
(b) One zygote divides and its early blastomeres separate, each developing into an embryo; both come from one sperm and one ovum (1 mark).
(c) No: both arise from one zygote with the same sex chromosomes (XX or XY), so they are of the same sex (1 mark).

Answer: (a) Fraternal: two ova, two sperms (X and Y). (b) Split of one zygote. (c) No, same zygote, same sex chromosomes.

Example 6: Significance of the trophoblast · 2 marks · Very short answerHarder board level

In the blastocyst, what is the fundamental significance of the trophoblast compared with the inner cell mass? What would happen if the trophoblast failed to attach?

Try it first, then show the solutionSolution

The trophoblast attaches the blastocyst to the endometrium (implantation) and later forms the chorionic villi of the placenta; the inner cell mass forms the embryo itself (1 mark).
Without attachment there is no implantation, so pregnancy is not established and the blastocyst is lost (1 mark).

Answer: Trophoblast = attachment and placenta; inner cell mass = embryo; no attachment → no pregnancy.

Where students lose marks in Fertilisation, Cleavage and Implantation

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

Mistake: Saying fertilisation occurs in the uterus
Students confuse the site of fertilisation with that of implantation.
Wrong working
Fertilisation takes place in the uterus.
Right working
Fertilisation: ampullary region of the fallopian tube; implantation: endometrium of the uterus.
Do this: Write both sites separately.
Usually costs: 1 mark.
Mistake: Calling cleavage meiosis
Any early division gets called meiosis.
Wrong working
The zygote undergoes meiotic cleavage.
Right working
Cleavage is mitotic; blastomeres are diploid.
Do this: Meiosis forms gametes; mitosis builds the embryo.
Usually costs: 1 mark.
Mistake: Saying the trophoblast forms the embryo
The two blastocyst parts are swapped.
Wrong working
Trophoblast differentiates into the embryo.
Right working
Inner cell mass forms the embryo; trophoblast attaches and helps form the placenta.
Do this: Trophoblast touches; inner cell mass = infant.
Usually costs: 1 mark.
Mistake: Leaving out meiosis II
Students write sperm + ovum → zygote straight away.
Wrong working
Sperm enters the secondary oocyte and they fuse into a zygote.
Right working
Sperm entry triggers meiosis II (second polar body + ootid); then the nuclei fuse.
Do this: Insert the step: 'completes meiosis II'.
Usually costs: 1 mark in 5-mark answers.

Exam tips

In 5-mark 'trace the events' answers, name the place of each event; marks are split by event.
Use the exact terms: ampullary region, zona pellucida, acrosome, blastomeres, morula, blastocyst, trophoblast, inner cell mass, endometrium.
In sex-determination answers, end with the conclusion: the father's sperm determines the sex.
Draw the blastocyst with the inner cell mass on one side, not in the centre.
Placenta, parturition and lactation need only an elementary idea; do not spend long on them in this topic.

Practice questions on Fertilisation, Cleavage and Implantation

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
Where does fertilisation occur in the human female?
(a) Uterus (b) Ampullary region (c) Cervix (d) Infundibulum
Hint
The wide part of the oviduct next to the infundibulum.
Answer
(b) Ampullary region.
Q2 · 1 mark · Assertion-reasonBoard level
Assertion (A): Not every copulation leads to fertilisation and pregnancy.
Reason (R): Fertilisation can occur only if the ovum and sperms are transported simultaneously to the ampullary region of the fallopian tube.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Timing.
Answer
(a). If the ovum and sperms do not reach the ampullary region at the same time, fertilisation cannot take place.
Q3 · 1 mark · MCQBoard level
Match: (a) Trophoblast (b) Implantation (c) Inner cell mass (d) Morula with (i) embedding of the blastocyst in the endometrium (ii) cells that differentiate into the embryo (iii) outer layer of the blastocyst (iv) 8-16 blastomeres.
Hint
One to one.
Answer
(a)-(iii), (b)-(i), (c)-(ii), (d)-(iv).
Q4 · 3 marks · Short answerHOTS / competency
Suppose a woman has a blocked isthmus in both fallopian tubes but healthy ovaries and uterus. (a) Can fertilisation still take place? (b) If it does, where would the embryo be unable to go, and why is this dangerous? Explain with reference to the normal path.
Hint
Fertilisation site vs path of the zygote.
Answer
(a) Sperms must pass the isthmus to reach the ampullary region, so with a complete block fertilisation is very unlikely; if the block is partial, sperms may get through. (b) A zygote formed would be unable to travel through the isthmus to the uterus; it normally cleaves while moving through the isthmus and implants in the endometrium. If it implants in the tube instead, the tube cannot support pregnancy and may rupture.
Q5 · 2 marks · Very short answerHarder board level
Explain why it is scientifically correct to say that the sex of a baby is determined by the father. What is the probability that a couple's fourth child is a girl if the first three are boys?
Hint
Independent events.
Answer
All ova carry X; half the sperms carry X and half Y. An X sperm gives XX (girl), a Y sperm XY (boy), so the sperm decides. Each birth is independent: probability of a girl = 1/2.
Q6 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose a diagram shows a human ovum surrounded by several sperms. One sperm has entered and its acrosome is no longer intact; a small cell is seen beside the ovum.
(a) Name the layer that the sperm crossed first. (1)
(b) What change in the ovum prevents the other sperms from entering? (1)
(c) Identify the small cell, and state what the entry of the sperm triggers in the ovum. (2)
Hint
Zona pellucida; meiosis II.
Answer
(a) Zona pellucida. (b) Contact of the sperm induces changes in the membrane that block entry of additional sperms. (c) A polar body; sperm entry triggers completion of meiosis II of the secondary oocyte, forming the haploid ovum and the second polar body, after which the nuclei fuse into a diploid zygote.

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.What is a morula?Show answer
An embryo with 8-16 blastomeres.
2.Which part of the blastocyst forms the embryo?Show answer
The inner cell mass.
3.Which layer of the ovum blocks extra sperms after first contact?Show answer
The zona pellucida (its membrane changes).
4.What triggers completion of meiosis II in the oocyte?Show answer
Entry of the sperm.
5.What is implantation?Show answer
Embedding of the blastocyst in the endometrium of the uterus.
6.Probability of a girl at any birth?Show answer
1/2.

Fertilisation, Cleavage and Implantation: frequently asked questions

Where does fertilisation take place in humans?

At the ampullary region of the fallopian tube.

How is polyspermy prevented?

When a sperm contacts the zona pellucida, it induces changes in the membrane that block the entry of additional sperms.

What is the difference between a morula and a blastocyst?

A morula is a solid ball of 8-16 blastomeres. A blastocyst is a later stage with an outer trophoblast and an inner cell mass, which implants in the uterus.

What is implantation?

The attachment of the blastocyst's trophoblast to the endometrium, after which uterine cells cover it and it becomes embedded. It leads to pregnancy.

Who determines the sex of a child?

The father. Ova carry only X; sperms carry X or Y, and the sperm that fertilises the ovum decides whether the zygote is XX or XY.