<P> The placenta occasionally takes a form in which it comprises several distinct parts connected by blood vessels . The parts, called lobes, may number two, three, four, or more . Such placentas are described as bilobed / bilobular / bipartite, trilobed / trilobular / tripartite, and so on . If there is a clearly discernible main lobe and auxiliary lobe, the latter is called a succenturiate placenta . Sometimes the blood vessels connecting the lobes get in the way of fetal presentation during labor, which is called vasa previa . </P> <P> About 20,000 protein coding genes are expressed in human cells and 70% of these genes are expressed in the normal mature placenta . Some 350 of these genes are more specifically expressed in the placenta and fewer than 100 genes are highly placenta specific . The corresponding specific proteins are mainly expressed in trophoblasts and have functions related to female pregnancy . Examples of proteins with elevated expression in placenta compared to other organs and tissues are PEG10 and the cancer testis antigen PAGE4 expressed in cytotrophoblasts, CSH1 and KISS1 expressed in syncytiotrophoblasts, and PAPPA2 and PRG2 expressed in extravillous trophoblasts . </P> <P> The placenta begins to develop upon implantation of the blastocyst into the maternal endometrium . The outer layer of the blastocyst becomes the trophoblast, which forms the outer layer of the placenta . This outer layer is divided into two further layers: the underlying cytotrophoblast layer and the overlying syncytiotrophoblast layer . The syncytiotrophoblast is a multinucleated continuous cell layer that covers the surface of the placenta . It forms as a result of differentiation and fusion of the underlying cytotrophoblast cells, a process that continues throughout placental development . The syncytiotrophoblast (otherwise known as syncytium), thereby contributes to the barrier function of the placenta . </P> <P> The placenta grows throughout pregnancy . Development of the maternal blood supply to the placenta is complete by the end of the first trimester of pregnancy week 14 (DM). </P>

When does the placenta implant in the uterus
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