• Mammals can be split into different categories on the basis of their?mechanism for foetal nourishment
  • Placental?mammals, such as humans
  • Monotremes, or egg laying mammals, such as a platypus
  • Marsupials,?such as kangaroos, whose offspring develop in a pouch
• Placental mammals?rely on the complex system of?blood vessels?specially designed to maximise exchange of substances between mother and foetus without any direct connection between them
• The placenta is also responsible for?production of key pregnancy hormones?oestrogen and progesterone
• The foetus is connected to the placenta via the?umbilical cord?and is contained within the?amniotic sac?filled with?amniotic fluid?which protects the foetus

Structure of the placenta

• The placenta is an organ primarily made up of a complex arrangement of?blood vessels arranged into placental,?or?chorionic, villi?with maternal blood flow distributed around the villi
  • Throughout the course of the pregnancy, the number of villi increases to meet the demands of the growing foetus
• Maternal blood and foetal blood never mix directly, but flow either side of a layer of cells that make up the placental barrier; there is a very short distance between the maternal and foetal blood to allow?exchange of nutrients and gases
  • The mother's blood flows?out of the mother's blood vessels?and forms pools in the spaces surrounding the placental villi; these spaces are known as the?inter-villous spaces
• The?placental membrane,?or?barrier,?provides a selectively permeable barrier which restricts the exchange of substances between mother and baby
• Substances that move across the barrier?from mother to foetus?include
  • Oxygen
  • Antibodies
    • Antibodies cross the placenta using a mechanism called?endocytosis
  • Water
  • Glucose
  • Unwanted?or?harmful substances?may also cross the placental barrier, including alcohol, drugs or small pathogens such as viruses
    • Bacterial pathogens are too large to cross the barrier
• Substances that move across the barrier?from foetus to mother?include
  • Carbon dioxide
  • Water
  • Urea
• The placenta is connected to the growing foetus by the?umbilical cord

The placenta brings the maternal and paternal blood systems close but without directly mixing

• Hormones play a fundamental role in?promoting foetal development?and?maintaining the pregnancy
• The levels of hormones change throughout the stages of pregnancy and development of pregnancy organs, such as the placenta
• Initially, the degenerating?corpus luteum?(a group of cells that form in the ovary following ovulation) releases hormones that support the initial stages of the pregnancy, up to about week 9
• The?placenta?then takes over in releasing steroid hormones,?oestrogen?and?progesterone
  • The role of?oestrogen?is to?stimulate growth?of the?uterus?and?mammary?glands?(commonly called breasts in humans)
  • The role of?progesterone?is to?support the lining of the uterus?to maintain the pregnancy
• If the placenta fails to initiate hormone production, a?miscarriage?may occur

Pregnancy hormones change throughout gestation and with placental growth