Identical twins start with two separate eggs that are fertilized by two different sperms....each has its own placenta and sac of water....and can be both a girl, a boy, both a girl and a boy....depending on where the eggs are implanted, the placentas can be completely separate or conjoined....then it splits into two..they are....this happens about 12 days after conception.... Identical twins are always of the same sex and feed from the same placenta (monochorion).... each embryo has its own umbilical cord... you will see 3 pregnancies with separate placentas and sacs.... vagina: you may have a little spotting at the end of this week, usually from blood implantation.. these cells will soon form the placenta... and now they produce the hormone HCG which makes the pregnancy test positive... blastocyst: each baby first has a It is a small ball of hundreds of cells that have the ability to quickly appear in different roles... (Blastocyst) The inner lining of the uterine cavity: the blastocyst (cell mass) has begun to penetrate into the wall of the uterine cavity.. Yolk sac: This cavity soon forms the yolk sac, which produces the baby's blood cells and is responsible for delivering oxygen and food to the baby until the placenta is placed inside the uterus in two layers.