<Table> <Tr> <Td> </Td> <Td> This article needs more medical references for verification or relies too heavily on primary sources . Please review the contents of the article and add the appropriate references if you can . Unsourced or poorly sourced material may be challenged and removed . (December 2017) </Td> <Td> </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article needs more medical references for verification or relies too heavily on primary sources . Please review the contents of the article and add the appropriate references if you can . Unsourced or poorly sourced material may be challenged and removed . (December 2017) </Td> <Td> </Td> </Tr> <Table> <Tr> <Th_colspan="2"> Follicular phase </Th> </Tr> <Tr> <Td_colspan="2"> Figure 1 . Menstrual cycle illustrating hormone levels, follicle development and uterine cycle </Td> </Tr> <Tr> <Th> Biological system </Th> <Td> Urogenital system (reproductive) </Td> </Tr> </Table>

How does the hypothalamus react to the level of estrogen at the beginning of the follicular phase