Revision Notes

16.2.2 Pregnancy & Birth

Growth & Development of the Fetus

  • After fertilisation in the oviduct, the zygote travels towards the uterus
  • This takes about 3 days, during which time the zygote will divide several times to form a ball of cells known as an embryo
  • In the uterus, the embryo embeds itself in the thick lining (implantation) and continues to grow and develop
  • The gestation period for humans is 9 months
  • Major development of organs takes place within the first 12 weeks, during which time the embryo gets nutrients from the mother by diffusion through the uterus lining
  • After this point the organs are all in place, the placenta has formed and the embryo is now called a fetus
  • The remaining gestation time is used by the fetus to grow bigger in size

The fetus in the uterus, IGCSE & GCSE Biology revision notesThe fetus in the uterus


  • The fetus is surrounded by an amniotic sac which contains amniotic fluid (made from the mother’s blood plasma)
  • This protects the fetus during development by cushioning it from bumps to the mother’s abdomen
  • The umbilical cord joins the fetus’s blood supply to the placenta for exchange of nutrients and removal of waste products
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The Placenta

  • During the gestation period the fetus develops and grows by gaining the glucose, amino acids, fats, water and oxygen it needs from the mother’s blood
  • The bloods run opposite each other, never mixing, in the placenta
  • The fetus’s blood connects to and from the placenta by the umbilical cord
  • The mother’s blood also absorbs the waste from the fetus’s blood in the placenta; substances like carbon dioxide and urea are removed from the fetus’s blood so that they do not build up to dangerous levels
  • Movement of all molecules across the placenta occurs by diffusion due to difference in concentration gradients
  • The placenta is adapted for this diffusion by having a large surface area and a thin wall for efficient diffusion
  • The placenta acts as a barrier to prevent toxins and pathogens getting into the fetus’s blood
  • Not all toxin molecules or pathogenic organisms (such as viruses, eg rubella) are stopped from passing through the placenta (this usually depends on the size of the molecule)
  • This is why pregnant women are advised not to smoke during pregnancy as molecules like nicotine can pass across the placenta
  • After the baby has been born, the umbilical cord is cut – this does not hurt as there are no nerves in it, just two blood vessels
  • It is tied off to prevent bleeding and shrivels up and falls off after a few days leaving the belly button behind
  • The placenta detaches from the uterus wall shortly after birth and is pushed out due to contractions in the muscular wall of the uterus – known as the afterbirth

The placenta, IGCSE & GCSE Biology revision notesThe placenta

Exam Tip

It is worth learning at least two specific substances that move in either direction across the placenta – this is a common exam question and non-specific answers such as ‘waste products’ and ‘nutrients’ will not get any marks!

Stages of Birth

  • Amniotic sac breaks
  • Muscles in the uterus wall contract
  • Cervix dilates (gets wider)
  • Baby passes out through the vagina
  • Umbilical cord is tied and cut
  • Afterbirth is delivered
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Antenatal Care

  • Antenatal (before birth), care is the name given to the care and advice given to expectant mothers along with checks on fetal growth and development
  • Whilst pregnant, expectant mothers are given advice on:
    • diet including the need to take folic acid to prevent developmental issues with the fetus and the importance of a balanced diet
    • exercise to stay fit
    • health precautions such as avoiding infections, tobacco, alcohol and other drugs
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  • During pregnancy the mammary glands enlarge and become prepared to secrete milk
  • Shortly after birth, the mother will be stimulated to release milk due to the sucking action of the baby at the breast
  • Some mothers struggle to breastfeed successfully and so may feed the baby using formula milk in a bottle

Comparing milk:

Breastfeeding table, IGCSE & GCSE Biology revision notes

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.

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