7Bd
Foetal development
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7Bd Gestation and birth
As soon as an egg cell is
fertilised, a baby will start to
develop.
Pregnancy lasts for 38 weeks.
After this gestation period, the
baby will have developed all the
organs and tissues it needs to
carry out all the life processes
outside its mother’s body.
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7Bd Gestation and birth
Timescale: at fertilisation.
 A single fertilised egg cell is formed.
 The cell starts to divide within 12–24 hours.
 The number of cells doubles every 12–24 hours to
produce an embryo.
protective
jelly layer
nucleus: formed from the nucleus of the
sperm cell and the nucleus of the egg cell.
Typical measurements:
Diameter: 0.1 mm
Mass: 4 millionths of a gram
cytoplasm: from the
original egg cell
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7Bd Gestation and birth
Timescale: 1 week after fertilisation.
 The embryo is a ball of 50 to 60 cells.
 It implants into the uterus lining at around this time.
 After implantation, parts of the embryo start to
develop into the placenta and umbilical cord.
Typical measurements:
Diameter: 1 mm
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7Bd Gestation and birth
Timescale: 5 weeks after fertilisation.
 The embryo has many recognisable features.
brain
eye
arm
umbilical
cord
spine
leg
Typical measurements:
Length: 4 mm
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7Bd Gestation and birth
Timescale: 9 weeks after fertilisation.
 Most organs are developing.
 The legs and arms start to move and the heartbeat may
just be heard from outside the mother’s body.
Typical measurements:
Length: 18 mm
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7Bd Gestation and birth
Timescale: 11 weeks after fertilisation.
 By this stage the placenta is fully functioning.
 The embryo is called a foetus, from now until birth.
 The head is nearly half the size of the foetus, and the
features of the face have developed.
Typical measurements:
Head circumference: 60 mm
Femur (upper leg bone) length: 10 mm
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7Bd Gestation and birth
Timescale: 15 weeks after fertilisation.
 An ultrasound scan may detect the sex of the foetus.
 The testes and penis have formed in males.
 The mouth of the foetus may show sucking movements.
Typical measurements:
Head circumference: 130 mm
Femur length: 22 mm
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7Bd Gestation and birth
Timescale: 24 weeks after fertilisation.
 The brain is developing rapidly.
 Other organs are well developed.
Typical measurements:
Head circumference: 220 mm
Femur length: 45 mm
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7Bd Gestation and birth
Timescale: 28 weeks after fertilisation.
 The brain is developing rapidly.
 Other organs are well-developed, and the fingers and
toes are fully formed.
Typical measurements:
Head circumference: 255 mm
Femur length: 55 mm
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7Bd Gestation and birth
Timescale: 34 weeks after fertilisation.
 Bones begin to harden.
 Body fat is increasing rapidly.
 There are rhythmic breathing movements that take
amniotic fluid into and out of the lungs.
Typical measurements:
Head circumference: 310 mm
Femur length: 68 mm
© Pearson Education Ltd 2014. Copying permitted for purchasing institution only. This material is not copyright free.
7Bd Gestation and birth
Timescale: 38 weeks after fertilisation.
 The foetus is fully developed and ready for birth.
Typical measurements:
Head circumference: 330 mm
Femur length: 75 mm
© Pearson Education Ltd 2014. Copying permitted for purchasing institution only. This material is not copyright free.