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Development of the foetus. Fertilisation - the joining of the sperm and the egg in the fallopian tube (below) to form a unique human being, occurs. .

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Development of the foetus

Fertilisation - the joining of the sperm and the egg in the fallopian tube (below) to form a unique human being, occurs. 


This is a fertilized egg only thirty hours after conception.  Magnified here, it is no larger than the head of a pin.  Still rapidly dividing, the developing embryo, called a zygote at this stage, floats down from the fallopian tube and towards the uterus.


WEEK 3-5 The embryo’s tiny heart begins to beat by day twenty-one.  Arm and leg buds are visible and the formation of the eyes, lips, and nose has begun. The spinal cord grows faster than the rest of the body giving a tail like appearance which disappears as the embryo continues to grow. The placenta begins to provide nourishment for the embryo.   


WEEK 7 Major organs have all begun to form.  The embryo has developed its own blood type, unique from the mother’s.  Hair follicles and knees and elbows are visible.  Facial features are also observable.  The eyes have a retina and lens.  The major muscle system is developed and the embryo is able to move.  


WEEK 8-12 The embryo is reactive to its environment inside the amniotic sac where it swims and moves.  Hands and feet can be seen.  At the end of week 8, the embryonic period is over and the foetal stage begins. 


Weeks 13-16 The brain is fully developed and the foetus can suck, swallow, and make irregular breathing sounds.  Foetus can feel pain.  Foetal skin is almost transparent.  Muscles tissue is lengthening and bones are becoming harder.    Liver and organs produce appropriate fluids.  Eyebrows and eyelashes appear and the fetus makes active movements including kicks and even somersaults. 


Week 20-24 A protective waxy substance called Vernix covers the skin.  By birth, most of the vernix will be gone but any that is left is quickly absorbed.  Foetus has a hand and  footprints and fingerprints are forming.  Foetus practices breathing by inhaling amniotic fluid into its developing lungs. 


Weeks 25 – 28 Rapid brain development occurs during this period and the nervous system is able to control some bodily functions.  The foetus’ eyelids now open and close.  At 25 weeks there is a 60% chance of survival if born.     

Weeks 29 – 32 There is a rapid increase in the amount of body fat the foetus has. Rhythmic breathing occurs, but the lungs are not yet mature.  The foetus sleeps 90-95% of the day.  At this point there the survival rate is above 95% if the baby is born. 


The Placenta

  • The placenta is an organ containing blood vessels that run very near to those of the mother.
  • The placenta is connected to the foetus by the umbilical cord.
  • Oxygen and food pass from the mother’s blood to the blood of the foetus.
  • Carbon dioxide and waste pass from the foetus to the mother.
  • Some harmful substances can cross the placenta.

7B Pregnancy - Dangers to fetus

Harmful substance

Harmful substance

Effect on baby

A Fetus may be infected with life-threatening disease before it is born.

4 HIV virus

1 alcohol

B Oxygen supply is reduced by carbon monoxide.

2 drugs

3 smoking

C Embryo infected and cannot develop properly. It may be born blind or deaf.

5 Rubella virus

3 smoking

2 drugs

D Blood vessels and heart may be damaged.

Baby maybe small, born early or born dead.

4 HIV virus

1 alcohol

5 Rubella virus

E Baby may be born underweight or brain-damaged.

Match the substance to the effect it may have on a developing baby.