Spiritually Adopted Babies Development: At 4 weeks the embryo's outer cells are tunneling into the lining of the mother’s uterus so blood flow from mom will be able to provide nutrients and oxygen to the growing baby. There's now a sac containing amniotic fluid to cushion the baby as he/she grows. The tiny embryo also has a yolk sac which temporarily provides nourishment and makes cells that turn into the umbilical cord, gastrointestinal system, and reproductive organs. Keep praying for your baby!