Edexcel IGCSE Biology: Double Science 复习笔记 3.1.6 Pregnancy

• After fertilisation in the oviduct, the zygote travels towards the uterus
• This usually takes about 3 days, during which time the zygote will divide several times to form a ball of cells known as an embryo
• Once it reaches the uterus, the embryo embeds itself in the thick lining of the uterus (a process known as implantation) and continues to grow and develop
• A placenta forms where the embryo is attached
  • The placenta is an organ that ensures materials can be exchanged between the blood of the mother and the blood of the growing embryo
  • The umbilical cord joins the embryo's blood supply to the placenta for exchange of nutrients and removal of waste products

   

• The embryo is surrounded by amniotic fluid (a fluid made from the mother’s blood plasma) that is held in by the amniotic membrane (also known as the amniotic sac)
  • The amniotic fluid protects the embryo during development by cushioning it from bumps to the mother’s abdomen

   

The foetus in the uterus

• For the next 9 months, the embryo grows in the uterus
  • This is known as the gestation period

   

• 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 fully formed and the embryo is now called a foetus (sometimes spelt fetus)
• The remaining gestation time is used by the foetus to grow bigger in size

• During the gestation period, the foetus develops and grows by gaining all the glucose, amino acids, fatty acids, water, oxygen, mineral ions and vitamins it needs from the mother’s blood
• In the placenta, the mother’s blood comes into very close proximity to the blood of the foetus but never mixes with it
• The foetus’s blood connects to and from the placenta by the umbilical cord
• The mother’s blood also absorbs the waste products from the foetus’s blood in the placenta; substances like carbon dioxide and urea are removed from the foetus’s blood so that they do not build up to dangerous levels
• The movement of most molecules across the placenta occurs by diffusion due to difference in concentration gradients
  • For some substances that need to be moved, active transport may be involved

   

• 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 foetus’s blood
• Not all toxin molecules or pathogenic organisms (such as viruses e.g. 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

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
  • This is known as the afterbirth