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Congratulations! One lucky sperm and your egg have come together in your fallopian tube! This fertilized ovum is now called a zygote, and will migrate down through your fallopian tube to your uterus. As it travels, it will undergo several rapid mitotic divisions, reaching the size of approximately 120 cells.
At this point, the embryo is now called a blastocyst. A blastocyst is made up of two cell types. It has an inner cell mass, or the embryoblast, which will form the embryo; and an outer cell mass, called the trophoblast, which forms the placenta. (Over the next nine months, the placenta will feed and nourish your baby, while also disposing of your baby's waste.) After reaching your uterus, the blastocyst will attach to the blood-rich tissue, called the endometrium, through an implantation process.
This is also when your baby's gender is determined, due to the sex chromosomes in the father's sperm. If two, or more, eggs are released from the ovaries, and then fertilized by two, or more, separate sperm, fraternal twins, or multiples, have been conceived. If one egg is fertilized and later divides in two, identical twins will develop (or divides into three, identical triplets will develop, etc).