Implantation takes place, when ovulation and fertilization occur. Implantation happens in early stage of pregnancy when the fertilized egg (zygote) travels down the fallopian tube to the uterus and attaches to the epithelium or uterine lining. It takes about 8 to 10 days for the fertilized egg to reaches to the uterus. During this time, it develops into a blastocyst through different stages of transformation beginning as a single cell dividing into 150 cells with an outer layer the trophoblast, a fluid-filled cavity the blastocoel, and a cluster of cells on the interior the inner cell mass. The tiny ball of cells is about a size of a poppy seed. It attaches to the epithelium during 4th week of gestation. Once it is firmly adheres, this is called as an embryo. The embryo then again divides into two parts which will become the placenta and the fetus. An ultrasound done during the 5 to 6 weeks of gestation period that may show the amniotic sac and yolk sac, which are forming during this time. The amniotic sac is where baby will develop. The yolk sac will later be incorporated in a baby’s digestive tract. This ultrasound confirms that implantation has taken place.