The placenta is a special outgrowth of the foetus that is firmly attached to the inside of the mother’s uterus (womb). It has blood vessels that penetrate into the wall of the uterus and interact with the mother’s arteries and veins to enable the foetus to draw oxygen and food from the mother’s system and send waste products to the mother for removal.
As the foetus grows, it floats in a fluid-filled sac like a water-filled balloon (the amniotic sac), and the foetus drinks the amniotic fluid and excretes into it through the kidneys. The amniotic sac and its fluid act as a very effective shock absorber so that the foetus can survive unharmed quite serious injuries to its mother (eg. a car crash). One side of the sac is especially modified into the placenta, while the rest is a fine but tough transparent membrane.
The foetus is connected to the placenta by the umbilical cord, which contains three intertwined blood vessels (a vein and two arteries), which convey nourishment from the mother to the foetus and waste products the other way. At birth, this is between 15 and 120 cm long and runs from the navel to the placenta, where the artery and veins it contains fan out to interact with the mother’s circulatory system.
The mother’s and baby’s blood streams remain separate and do not mingle. Doctors will check the cord after birth, and if only one vein is present instead of two, it is probable that the baby will have some hidden birth defect.
The placenta is a flat, circular organ consisting of a spongy network of blood vessels. It acts as a combined lung, liver, kidney and digestive tract for the developing foetus. Oxygen, nutrients, waste products and other substances (eg. alcohol and some drugs) can pass freely through the placenta from the bloodstream of the mother to the bloodstream of the foetus. Infections (particularly viruses such as German measles) may also pass to the foetus through the placenta.
Several minutes after the birth, the placenta (the afterbirth) is expelled by further contractions of the uterus, assisted by gentle traction on the cord by the doctor or midwife. Occasionally the placenta may not be expelled, which leads to intervention by a doctor.