Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels.

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Many more pregnancy stages and
other anatomical models
available online
via this link

Events after fertilisation

At fertilisation, a sperm and an egg meet and their nuclei fuse. This means that a "new" cell is produced, which combines genetic information from the father and the mother ( in fact, exactly half from each).
See later notes on genetics.

This cell - the zygote - divides again and again to form a ball of cells, called an embryo , which eventually becomes attached to the lining of the uterus (also called the womb) - a process called implantation .
About how many cells make up the embryo at this stage?
> 100
Why is this about the same size as the (single) fertilised egg?
> nothing added - all from division of one cell

The embryo also releases hormones which prevent the normal events of the menstrual cycle, so that the lining of the uterus is not broken down, and stabilise conditions inside the uterus for the duration of pregnancy. This means that a pregnant woman will not undergo menstruation, so "missing a period" is one of the first signs of pregnancy. The hormones will also be present in a pregnant woman's urine, and this is the basis of most pregnancy tests.

As time proceeds, the embryo develops into a stage with noticeably human features, which is known as a foetus , but of course it is also called a baby, even before it emerges after about 9 (calendar) months!

Part of the embryo develops into a structure called the placenta, which has a vital role in supplying everything that the developing embryo needs, and removing what it does not require. As such it maintains constant internal conditions - homeostasis. Actually the placenta is thought to be the main reason for the reproductive success of the main group of mammals. It is also called the after-birth, for reasons which will be obvious later!

Name some (specific) substances that an embryo requires.
> glucose, amino acids etc. ("foods")> oxygen

Name some waste substances that an embryo does not require.
> carbon dioxide > urea

All these substances are in fact supplied or removed in solution, by the mother's blood circulation system working in conjunction with the developing baby's own blood system. However, the mother's and the baby's blood do not mix, but they come into very close contact in the placenta.
Blood flows back and forth between the placenta and the embryo along blood vessels inside the umbilical cord , and it is protected inside a bag of fluid called the amnion .
What causes the blood to flow along these blood vessels?
> embryonic heart

Development of the baby

Zygote to Foetus
Day 1
Day 1: Fertilisation : 0.14 mm
Day 2

Day 2: Zygote divides : 0.14 mm
Day 3
Day 3: Morula : 0.14 mm

Day22: Heart starts to beat. 2.0 mm
Day 29: Crown to rump : 5.0 mm
Day 29: Crown to rump : 12.0-14.0 mm
Day 29: Crown to rump : 23.0-23.5 mm
A new human life is started (conceived) in the zygote at the moment of fertilisation, but in the resulting ball of cells there is practically no difference between the cells.
As it grows into an embryo - the cells become specialised for various functions (differentiation) as different body structures become distinct.
From about 8 weeks onwards, it is technically called a foetus. More and more features of the human body become noticeable, even though the foetus itself is small. As the foetus grows, the mother's uterus expands. Usually, the foetus eventually ends up in a head downwards position, and this needs to be checked.
At ante-natal clinics, other checks are carried out in order to monitor the progress of the pregnancy.
These usually include:
- externally checking the growth of the developing foetus by measuring the size of the mother's abdomen
- using a stethoscope to listen to the baby's heartbeat
- monitoring of mother's blood pressure and blood haemoglobin
- possibly more advanced techniques such as ultrasound scanning, which produces a "picture" of the baby in the uterus.

If required, cells may be removed from the liquid surrounding the embryo (amniocentesis) or from the placenta (chorionic villi sampling-CVS), and these may be used in screening for certain genetic diseases. These are fairly simple operations, but not without risks to the developing foetus.

It is also worth noting that these services are only available in the developed countries.
Elsewhere, childbirth remains more problematic.

The development of the baby
6 weeks
10 weeks
21 weeks
a few weeks before birth
A pregnant woman needs to take special care of her health, because this also has a great influence on the health of the baby. She must be careful about her diet and other activities. In particular, some disease organisms and chemical substances can pass across the placenta and affect the development process.
Name some diseases or chemical substances that can do this, and briefly describe their effects.

Disease organism or chemical
substance crossing placenta
Possible effect on baby
german measles (Rubella) nervous system problems -
smaller birth weight
CO/ nicotine from
smaller birth weight
birth defects - deformed/reduced


About 1 in 80 pregnancies result in twins being born. Usually, twins are formed as a result of a woman releasing two separate eggs at ovulation, each of which is fertilised by different sperms. The resulting zygotes independently go through all the same stages, and two embryos become implanted at different places in the uterus. These dizygotic or fraternal twins may be of the same sex or different, and are no more alike than normal brothers and sisters, because they share the same mother and father, but inherit different combinations of genes from each.
Identical (monozygotic) twins (1 in 276 pregnancies) result from a single zygote, the resulting ball of cells separating into two portions which again become independently implanted. However, the resulting individuals are always of the same sex, and very similar appearance because they have exactly the same genetic makeup.
Siamese twins, which are very rare, probably result from incomplete separation of two masses of cells, so that the resulting babies are born joined together in some way, and possibly sharing internal organs. To remedy the situation surgically may present a difficult ethical dilemma.


Pregnancy normally lasts 38 weeks from conception, or 40 weeks (280 days), counting from the last period. This stage is called (full) term, but premature babies may be born before this. The average pregnancy represents a chance for a foetus to develop as far as possible in a protected environment, so to some extent the larger the baby grows the better its chances of survival, but the size of the baby's head is effectively a limiting factor.
According to their size, survival of premature babies may depend on medical technology (perhaps requiring incubators to maintain body temperature, and ventilators to assist breathing. Overdue babies may place their mothers' health under strain.

The events during the birth process are probably triggered by changes in the level of hormones in the mother's body; in fact birth is sometimes induced by the administration of hormones if there is thought to be a medical reason, such as the likelihood of stress in mother or baby.
Labour is brought on by repeated contractions of the muscles of the uterus, which increase in strength and frequency as the baby is pushed out of the uterus, through the cervix and down the vagina. As this starts, the bag of liquid called the amnion breaks, so a sign of imminent birth is when the "waters burst". Throughout the birth, the baby continues to be attached via the umbilical cord and placenta to the mother's body systems. Complications may arise if birth takes a long time, or if the umbilical cord is constricted. When born, the baby can breathe air, once its lungs have been cleared of fluid, and the umbilical cord is then clamped and cut. A short time later, the placenta is also expelled from the uterus, as the after-birth.

It is considered normal for these events to be under the medical supervision of a team including a doctor and midwife.

Depending on the size or orientation of the foetus, or the health of the mother, it may decided to opt for a Caesarean section, which involves surgery to open up the abdomen wall and remove the baby, rather than the normal birth process.

Care of the new-born baby

Babies must be looked after carefully, kept clean and warm, and fed with milk. Human mother's milk is preferable to prepared substitutes based on cow's milk or other substances.
Draw up a table below to list the advantages of these two.<
Breast feeding Bottle feeding
suited to baby's needs anyone can do it - even Dad !
- creating bond between father and baby?
free from "germs" more convenient? - measured amounts supplied
free (from expense)? less embarassing in public?
more easily digestible baby may be more satisfied/last longer between feeds
less likelihood of allergies mother can be more independent, return to work etc
gives natural immunity

After 4-6 months, the baby is gradually switched to a more solid diet (weaned), so cereals and other foods are usually introduced.

Development of the human child takes a long time compared with other species of mammals, and involves much care from parents and the community in general.

Pregnancy models available online

Mind boggling!


?? Charts
Many other anatomical models and charts available by following this link.

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