Between four and seven days after fertilization has occurred, your fertilized egg will have reached the uterus and will now have divided into hundreds of tiny cells.
Soon after fertilisation, the zygote (the one-celled entity formed when sperm and egg unite) travels down the fallopian tube towards the uterus. It’s pretty tiny — about the size of a grain of salt. The zygote will begin rapidly dividing to form a cluster of cells. The inner group will become the embryo, the outer group will develop into the membranes and the placenta that nourish and protect your baby. During this first week, your baby is about 0.150mm long.
Implantation usually occurs in the upper part of your uterus. The blastocyst secretes hormones, which helps it to implant itself in the endometrium (lining of your uterus). These hormones also stop your body from releasing the endometrium, resulting in your first missed period. Implantation is a critical part of the development process as in these very early stages the endometrium provides your embryo with nutrients, and removes waste.
Once implanted, the outer layer of your blastocyst will connect with your small blood vessels to form the placenta. The inner cells will continue to develop and form an embryo.