A milestone in your baby's development-the placenta takes over from the yolk sac to provide your baby with nutrients. Just like your baby, the placenta has needed to grow and develop a circulation to support the ever-increasing demands that are being placed on it.
One week into the pregnancy, the placenta formed a distinct inner and outer layer of cells that gradually penetrated the lining of the uterus, with fingerlike fronds. You may have noticed a very slight bleed at this implantation stage (see You are 3 Weeks and 5 Days). More and more fronds spread out into the lining of the uterus, which itself undergoes a transformation process that enables each frond or villus to be bathed by small pools of maternal blood, enabling oxygen and nutrient transfer to take place.
Up until now, this blood flow has been limited by plugs of tissue, but at this stage of pregnancy these plugs begin to disappear. This means the placenta is sufficiently developed to withstand the pressure of maternal blood on each delicate villous. Villi will continue to branch out until around 30 weeks of pregnancy.