• 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