After implantation is complete in your uterus, the cells will split into two groups. One half will become the placenta. This is a special tissue that delivers must-have nutrients and oxygen to your baby throughout your entire pregnancy. The other half forms the embryo, and continues to grow. At this stage, we have the creation of the neural tube – where the baby’s brain, spinal cord and backbone will ultimately form. Two other structures that form at this time are called the amnion and yolk sac. The amnion is filled with amniotic fluid, and it will surround and protect the growing embryo. The yolk sac produces blood and nourishes the embryo with required nutrients (until the placenta fully forms and takes over that role).
Despite its extremely tiny size (around one millimeter), the embryo has three distinct layers of cells. The inner layer – the endoderm, develops into your baby’s digestive system, liver and lungs. The middle layer – the mesoderm develops into the baby’s heart, sex organs, bones, kidneys and muscles. The outer layer –the ectoderm, will form your baby’s nervous system, hair, skin and eyes.