A Baby Is Fed for Nine Months Without Sharing a Drop of Blood
Learn where fertilisation happens and what the zygote does next, how implantation works and what the placenta exchanges, the stages from zygote to foetus with the gestation period, and how the menstrual cycle relates to ovulation.
How is a developing baby fed if its blood never mixes with its mother's?
Through the placenta, which lets substances cross without letting the two bloodstreams meet.
The mother's blood and the foetus's blood flow through separate vessels, separated by a very thin membrane. Oxygen and nutrients diffuse across that membrane from mother to foetus, and carbon dioxide and waste diffuse back the other way — but the blood itself stays on its own side throughout.
The reason this matters is that the two bloods may be of different groups, and mixing them could be harmful. So the placenta is not merely a feeding tube; it is a carefully selective exchange surface. This page covers the second part of the ICSE Class 8 Biology chapter on reproduction in humans.
The mother's blood and the foetus's blood flow through separate vessels, separated by a very thin membrane. Oxygen and nutrients diffuse across that membrane from mother to foetus, and carbon dioxide and waste diffuse back the other way — but the blood itself stays on its own side throughout.
The reason this matters is that the two bloods may be of different groups, and mixing them could be harmful. So the placenta is not merely a feeding tube; it is a carefully selective exchange surface. This page covers the second part of the ICSE Class 8 Biology chapter on reproduction in humans.
Where does fertilisation happen, and what does the zygote do next?
In the fallopian tube — specifically in its upper part, not in the uterus.
Fertilisation is the fusion of the nucleus of a sperm with the nucleus of an ovum. The cell it produces is the zygote, and it is the first cell of a new individual.
The sequence:
- Semen is deposited in the vagina, and sperm swim up through the cervix and the uterus into the fallopian tube.
- Enormous numbers set out and very few arrive — the journey is long relative to a sperm's size.
- One sperm penetrates the ovum. The moment it does, the ovum's surface changes so no other sperm can enter, which is why a zygote has exactly two sets of genetic material and not three.
- The two nuclei fuse, forming the zygote.
The chromosome arithmetic. A human body cell has 46 chromosomes, in 23 pairs. Each gamete carries only 23 — half the number — so when they fuse:
and the zygote has the full complement again, with 23 from each parent. This is why a child inherits equally from both, and why the gamete must carry half and not the whole number.
What the zygote does next. It begins to divide — one cell into two, two into four, and so on — while being carried slowly along the fallopian tube towards the uterus. It does not grow larger at first; it simply becomes a ball of many smaller cells, reaching the uterus after about a week.
The boundary case worth knowing. If the dividing ball splits completely into two at an early stage, each part develops into a separate individual — identical twins, from one zygote and therefore genetically the same. If two ova are released and each is fertilised by a different sperm, the result is non-identical twins, who are no more alike than any two siblings.
Fertilisation is the fusion of the nucleus of a sperm with the nucleus of an ovum. The cell it produces is the zygote, and it is the first cell of a new individual.
The sequence:
- Semen is deposited in the vagina, and sperm swim up through the cervix and the uterus into the fallopian tube.
- Enormous numbers set out and very few arrive — the journey is long relative to a sperm's size.
- One sperm penetrates the ovum. The moment it does, the ovum's surface changes so no other sperm can enter, which is why a zygote has exactly two sets of genetic material and not three.
- The two nuclei fuse, forming the zygote.
The chromosome arithmetic. A human body cell has 46 chromosomes, in 23 pairs. Each gamete carries only 23 — half the number — so when they fuse:
and the zygote has the full complement again, with 23 from each parent. This is why a child inherits equally from both, and why the gamete must carry half and not the whole number.
What the zygote does next. It begins to divide — one cell into two, two into four, and so on — while being carried slowly along the fallopian tube towards the uterus. It does not grow larger at first; it simply becomes a ball of many smaller cells, reaching the uterus after about a week.
The boundary case worth knowing. If the dividing ball splits completely into two at an early stage, each part develops into a separate individual — identical twins, from one zygote and therefore genetically the same. If two ova are released and each is fertilised by a different sperm, the result is non-identical twins, who are no more alike than any two siblings.
What is implantation, and what does the placenta do?
Implantation is the embedding of the ball of cells into the thick, soft lining of the uterus.
The lining has been thickening and developing a rich blood supply in preparation, which is exactly what the menstrual cycle of the last section arranges. The ball of cells sinks into it and attaches, and from this point the developing individual is called an embryo.
The placenta then develops — a disc-shaped structure on the uterine wall, connected to the embryo by the umbilical cord, which carries blood vessels between them.
What the placenta does:
- Supplies oxygen from the mother's blood to the foetus
- Supplies nutrients — glucose, amino acids, minerals and vitamins
- Removes carbon dioxide and nitrogenous waste from the foetus into the mother's blood, for her lungs and kidneys to dispose of
- Acts as a barrier, keeping out many germs and harmful substances
- Secretes hormones that maintain the pregnancy
How the exchange works. The mother's blood and the foetus's blood run in separate vessels, separated by a thin membrane. Substances cross by diffusion, from wherever they are more concentrated to wherever they are less. Oxygen is more concentrated in the mother's blood, so it moves in; carbon dioxide is more concentrated in the foetal blood, so it moves out. The two bloodstreams never mix.
Two more protective structures:
- The amnion, a membrane enclosing the foetus, and the amniotic fluid inside it, which cushions the foetus against shocks and jolts and keeps its temperature steady.
- The muscular uterine wall, which can stretch enormously as the foetus grows.
Why the barrier is only partial. The placenta stops many harmful things but not all. Alcohol, nicotine, certain drugs and some viruses can cross it, which is the medical reason these are avoided during pregnancy. Describing the placenta as a complete barrier is a common overstatement, and the accurate answer is that it filters rather than seals.
Note which organs are doing the foetus's work. The foetus does not breathe, eat or excrete for itself. The mother's lungs, digestive system and kidneys serve both, with the placenta as the link — which is why a pregnant woman's requirements of food, iron and oxygen are higher than usual.
The lining has been thickening and developing a rich blood supply in preparation, which is exactly what the menstrual cycle of the last section arranges. The ball of cells sinks into it and attaches, and from this point the developing individual is called an embryo.
The placenta then develops — a disc-shaped structure on the uterine wall, connected to the embryo by the umbilical cord, which carries blood vessels between them.
What the placenta does:
- Supplies oxygen from the mother's blood to the foetus
- Supplies nutrients — glucose, amino acids, minerals and vitamins
- Removes carbon dioxide and nitrogenous waste from the foetus into the mother's blood, for her lungs and kidneys to dispose of
- Acts as a barrier, keeping out many germs and harmful substances
- Secretes hormones that maintain the pregnancy
How the exchange works. The mother's blood and the foetus's blood run in separate vessels, separated by a thin membrane. Substances cross by diffusion, from wherever they are more concentrated to wherever they are less. Oxygen is more concentrated in the mother's blood, so it moves in; carbon dioxide is more concentrated in the foetal blood, so it moves out. The two bloodstreams never mix.
Two more protective structures:
- The amnion, a membrane enclosing the foetus, and the amniotic fluid inside it, which cushions the foetus against shocks and jolts and keeps its temperature steady.
- The muscular uterine wall, which can stretch enormously as the foetus grows.
Why the barrier is only partial. The placenta stops many harmful things but not all. Alcohol, nicotine, certain drugs and some viruses can cross it, which is the medical reason these are avoided during pregnancy. Describing the placenta as a complete barrier is a common overstatement, and the accurate answer is that it filters rather than seals.
Note which organs are doing the foetus's work. The foetus does not breathe, eat or excrete for itself. The mother's lungs, digestive system and kidneys serve both, with the placenta as the link — which is why a pregnant woman's requirements of food, iron and oxygen are higher than usual.
What are the stages from zygote to birth?
Zygote, then embryo, then foetus, then birth — and the whole process takes about nine months.
- Zygote — the single cell formed at fertilisation, which begins dividing at once.
- Ball of cells — the dividing zygote travelling down the fallopian tube, reaching the uterus in about a week and implanting.
- Embryo — from implantation to roughly the eighth week. During this stage the cells begin to differentiate: the major organs — heart, brain, liver, limbs — start to form, and the heart begins to beat. The embryo is still very small.
- Foetus — from about the ninth week onwards. By now the form is recognisably human, with the organs already laid down. The rest of the time is spent on growth and on the organs maturing enough to work independently.
- Birth, also called parturition — the muscular wall of the uterus contracts powerfully and pushes the baby out through the cervix and vagina. The umbilical cord is cut, and the newborn breathes for itself for the first time.
The gestation period — the time from fertilisation to birth — is about 280 days in humans, which is roughly forty weeks or nine months.
The distinction that gets tested. Embryo and foetus are not interchangeable words. Up to about eight weeks the organs are being formed, and the term is embryo. After that they are being grown and matured, and the term is foetus. The switch marks a change in what is happening, not just a change of name.
Why the embryo stage is the delicate one. Organs are being built during it, so anything that interferes then can affect how an organ forms. Once the foetus stage is reached the structures already exist and are mainly enlarging, which is a different and less fragile process.
Gestation varies enormously between animals, and it tracks size and development at birth. A mouse needs only a few weeks; an elephant needs far longer than a human. A newborn that must walk almost immediately needs more development before birth than one that will be carried.
- Zygote — the single cell formed at fertilisation, which begins dividing at once.
- Ball of cells — the dividing zygote travelling down the fallopian tube, reaching the uterus in about a week and implanting.
- Embryo — from implantation to roughly the eighth week. During this stage the cells begin to differentiate: the major organs — heart, brain, liver, limbs — start to form, and the heart begins to beat. The embryo is still very small.
- Foetus — from about the ninth week onwards. By now the form is recognisably human, with the organs already laid down. The rest of the time is spent on growth and on the organs maturing enough to work independently.
- Birth, also called parturition — the muscular wall of the uterus contracts powerfully and pushes the baby out through the cervix and vagina. The umbilical cord is cut, and the newborn breathes for itself for the first time.
The gestation period — the time from fertilisation to birth — is about 280 days in humans, which is roughly forty weeks or nine months.
The distinction that gets tested. Embryo and foetus are not interchangeable words. Up to about eight weeks the organs are being formed, and the term is embryo. After that they are being grown and matured, and the term is foetus. The switch marks a change in what is happening, not just a change of name.
Why the embryo stage is the delicate one. Organs are being built during it, so anything that interferes then can affect how an organ forms. Once the foetus stage is reached the structures already exist and are mainly enlarging, which is a different and less fragile process.
Gestation varies enormously between animals, and it tracks size and development at birth. A mouse needs only a few weeks; an elephant needs far longer than a human. A newborn that must walk almost immediately needs more development before birth than one that will be carried.
How is the menstrual cycle related to ovulation?
Menstruation happens about two weeks after ovulation, if the released ovum was not fertilised.
The menstrual cycle is the roughly 28-day cycle of changes in the female reproductive system, controlled by hormones from the ovaries and the pituitary gland.
The cycle in outline, counting from the first day of bleeding:
- Days 1 to about 5 — menstruation. The thickened lining of the uterus, no longer needed, breaks down and is shed along with some blood through the vagina.
- Days 6 to 13 — the uterine lining thickens again and develops a rich blood supply, preparing to receive an embryo. Meanwhile an ovum matures in an ovary.
- About day 14 — ovulation. One ovum is released from an ovary into the fallopian tube. This is the only time in the cycle when fertilisation is possible.
- Days 15 to 28 — the lining is kept thick and ready. Then one of two things happens.
If the ovum is fertilised — it divides, travels to the uterus and implants in that prepared lining. The lining is retained, menstruation does not occur, and pregnancy begins. This is why a missed period is the first sign of pregnancy.
If the ovum is not fertilised — it degenerates. The lining is no longer required, so it breaks down and is shed, and the next cycle begins from day one.
So the relationship is a preparation and a clearing-up. The first half of the cycle prepares the uterus and releases an ovum; the second half waits; and menstruation is what happens when the preparation turns out not to have been needed.
Two terms for the beginning and the end. The first menstruation, occurring at puberty, is menarche. The permanent stopping of the cycles in middle age is menopause, after which a woman can no longer conceive.
A caution about the numbers. The 28-day cycle and day-14 ovulation are averages, not fixed rules. Real cycles vary considerably between individuals and from month to month, and an irregular cycle — particularly in the first years after menarche — is common and usually normal.
The menstrual cycle is the roughly 28-day cycle of changes in the female reproductive system, controlled by hormones from the ovaries and the pituitary gland.
The cycle in outline, counting from the first day of bleeding:
- Days 1 to about 5 — menstruation. The thickened lining of the uterus, no longer needed, breaks down and is shed along with some blood through the vagina.
- Days 6 to 13 — the uterine lining thickens again and develops a rich blood supply, preparing to receive an embryo. Meanwhile an ovum matures in an ovary.
- About day 14 — ovulation. One ovum is released from an ovary into the fallopian tube. This is the only time in the cycle when fertilisation is possible.
- Days 15 to 28 — the lining is kept thick and ready. Then one of two things happens.
If the ovum is fertilised — it divides, travels to the uterus and implants in that prepared lining. The lining is retained, menstruation does not occur, and pregnancy begins. This is why a missed period is the first sign of pregnancy.
If the ovum is not fertilised — it degenerates. The lining is no longer required, so it breaks down and is shed, and the next cycle begins from day one.
So the relationship is a preparation and a clearing-up. The first half of the cycle prepares the uterus and releases an ovum; the second half waits; and menstruation is what happens when the preparation turns out not to have been needed.
Two terms for the beginning and the end. The first menstruation, occurring at puberty, is menarche. The permanent stopping of the cycles in middle age is menopause, after which a woman can no longer conceive.
A caution about the numbers. The 28-day cycle and day-14 ovulation are averages, not fixed rules. Real cycles vary considerably between individuals and from month to month, and an irregular cycle — particularly in the first years after menarche — is common and usually normal.
Exam tip
Exam tip: state that the two bloodstreams do not mix
When describing the placenta, say explicitly that the mother's and foetus's blood do not mix and that substances cross by diffusion through a thin membrane. That sentence is worth its own mark.
List the placenta's functions in both directions — oxygen and nutrients in, carbon dioxide and nitrogenous waste out — plus its barrier and hormone roles. Giving only the feeding half is half an answer.
Keep embryo and foetus separate: embryo up to about eight weeks while organs form, foetus afterwards while they grow and mature.
Quote the gestation period as about 280 days, or nine months, and call birth parturition.
Say fertilisation occurs in the fallopian tube and implantation in the uterus. These are two different places and two different events.
For the menstrual cycle, give the day numbers: menstruation around days 1 to 5, ovulation about day 14, cycle length about 28 days — and add that these are averages.
State clearly that menstruation occurs only if the ovum is not fertilised, and that its absence is the first sign of pregnancy.
Name menarche and menopause, and give the chromosome arithmetic if asked why a gamete carries half.
List the placenta's functions in both directions — oxygen and nutrients in, carbon dioxide and nitrogenous waste out — plus its barrier and hormone roles. Giving only the feeding half is half an answer.
Keep embryo and foetus separate: embryo up to about eight weeks while organs form, foetus afterwards while they grow and mature.
Quote the gestation period as about 280 days, or nine months, and call birth parturition.
Say fertilisation occurs in the fallopian tube and implantation in the uterus. These are two different places and two different events.
For the menstrual cycle, give the day numbers: menstruation around days 1 to 5, ovulation about day 14, cycle length about 28 days — and add that these are averages.
State clearly that menstruation occurs only if the ovum is not fertilised, and that its absence is the first sign of pregnancy.
Name menarche and menopause, and give the chromosome arithmetic if asked why a gamete carries half.
Did you know
Why does a newborn cry almost immediately?
For nine months the foetus received all its oxygen through the placenta, dissolved in the mother's blood and diffusing across into its own. Its lungs were fully formed but had never been used, and were collapsed and filled with fluid.
At birth the umbilical cord is cut, and the placenta supply stops within moments. The newborn now has one option: use its lungs.
That first cry is the first breath. Crying draws air in forcefully enough to inflate the collapsed air sacs for the first time and to clear the fluid out of them — which a gentle breath could not manage. It is a mechanical necessity, not a complaint.
It is also why the absence of that first cry is treated as urgent by anyone attending a birth. The switch from placental oxygen to lung oxygen has to happen in the first minute or so, and the cry is the audible evidence that it has.
At birth the umbilical cord is cut, and the placenta supply stops within moments. The newborn now has one option: use its lungs.
That first cry is the first breath. Crying draws air in forcefully enough to inflate the collapsed air sacs for the first time and to clear the fluid out of them — which a gentle breath could not manage. It is a mechanical necessity, not a complaint.
It is also why the absence of that first cry is treated as urgent by anyone attending a birth. The switch from placental oxygen to lung oxygen has to happen in the first minute or so, and the cry is the audible evidence that it has.
Key takeaways
Fertilisation, the placenta and the menstrual cycle: quick revision
- Fertilisation is the fusion of a sperm nucleus with an ovum nucleus, and it occurs in the fallopian tube. The product is the zygote.
- Only one sperm enters; the ovum's surface then blocks the rest.
- A body cell has 46 chromosomes in 23 pairs; each gamete carries 23, so in the zygote, with half from each parent.
- The zygote divides into a ball of cells while travelling down the tube, reaching the uterus in about a week.
- Identical twins come from one zygote splitting; non-identical twins from two ova fertilised by two sperm.
- Implantation is the embedding of the ball of cells in the thick uterine lining; after it, the individual is an embryo.
- The placenta on the uterine wall connects to the embryo by the umbilical cord. It supplies oxygen and nutrients, removes carbon dioxide and nitrogenous waste, acts as a partial barrier, and secretes hormones.
- The two bloodstreams never mix; exchange is by diffusion across a thin membrane. The barrier is partial — alcohol, nicotine and some viruses can cross.
- The amnion and amniotic fluid cushion the foetus and keep its temperature steady.
- Stages: zygote ball of cells embryo (to about 8 weeks, organs forming) foetus (from about 9 weeks, organs growing and maturing) birth (parturition), when the uterine muscles contract.
- Gestation period: about 280 days, roughly nine months.
- Menstrual cycle, about 28 days: menstruation days 1 to 5; lining thickens days 6 to 13; ovulation about day 14; then the lining is held ready.
- If fertilised, the embryo implants and menstruation does not occur. If not, the ovum degenerates and the lining is shed.
- Menarche is the first menstruation; menopause is when cycles stop permanently. All the day numbers are averages.
Try drawing the 28-day cycle as a timeline with ovulation marked, then listing the placenta's functions in both directions — those two answers together cover most of this chapter.
- Only one sperm enters; the ovum's surface then blocks the rest.
- A body cell has 46 chromosomes in 23 pairs; each gamete carries 23, so in the zygote, with half from each parent.
- The zygote divides into a ball of cells while travelling down the tube, reaching the uterus in about a week.
- Identical twins come from one zygote splitting; non-identical twins from two ova fertilised by two sperm.
- Implantation is the embedding of the ball of cells in the thick uterine lining; after it, the individual is an embryo.
- The placenta on the uterine wall connects to the embryo by the umbilical cord. It supplies oxygen and nutrients, removes carbon dioxide and nitrogenous waste, acts as a partial barrier, and secretes hormones.
- The two bloodstreams never mix; exchange is by diffusion across a thin membrane. The barrier is partial — alcohol, nicotine and some viruses can cross.
- The amnion and amniotic fluid cushion the foetus and keep its temperature steady.
- Stages: zygote ball of cells embryo (to about 8 weeks, organs forming) foetus (from about 9 weeks, organs growing and maturing) birth (parturition), when the uterine muscles contract.
- Gestation period: about 280 days, roughly nine months.
- Menstrual cycle, about 28 days: menstruation days 1 to 5; lining thickens days 6 to 13; ovulation about day 14; then the lining is held ready.
- If fertilised, the embryo implants and menstruation does not occur. If not, the ovum degenerates and the lining is shed.
- Menarche is the first menstruation; menopause is when cycles stop permanently. All the day numbers are averages.
Try drawing the 28-day cycle as a timeline with ovulation marked, then listing the placenta's functions in both directions — those two answers together cover most of this chapter.