The uterus lining (endometrium) changes to ensure that if an egg is fertilized, it has a blood-rich place to implant.
Days 1-5 is menstruation. If fertilization did not occur in the previous cycle, the thick uterus lining breaks down and is lost through the vagina, a period.
Days 6-14 is repair and thickening. Stimulated by oestrogen, the lining starts to thicken again and its blood vessels regrow to prepare for a possible embryo.
Days 15-28 is maintenance. Stimulated by progesterone, the lining is maintained at its full thickness. It becomes more filled with blood vessels to provide nutrients for an implanted embryo.
Changes in the ovaries:
The ovaries focus on the maturation and release of the female gamete (egg).
Days 1-13 is the follicular phase. Inside the ovary, a follicle, which is a sac containing an immature egg, begins to develop and mature. This is triggered by Follicle Stimulating Hormone (FSH).
Day 14 is ovulation. A surge in Luteinizing Hormone (LH) causes the mature follicle to burst, releasing the egg from the ovary into the oviduct.
Days 15-28 is the luteal phase. The empty follicle transforms into a structure called the yellow body.
If no pregnancy occurs, the yellow body degenerates, progesterone levels drop, and the cycle restarts with menstruation.
The production sites change as the follicle develops and then transforms after ovulation.
Oestrogen is produced by the ovaries, specifically by the maturing follicles. Its role is to stimulate the repair and thickening of the uterus during the first half of the cycle.
Progesterone is produced by the yellow body (left behind in the ovary after egg is released). Its role is to maintain the thickness and blood supply of the uterus during the second half of the cycle.
If an embryo implants, the body must prevent menstruation, which requires keeping progresterone and oestrogen levels high.
Early pregnancy, first trimester:
The yellow body continues to produce oestrogen and progesterone. It is “saved” from breaking down by hormones released by the developing embryo.
Late pregnancy, four months onward:
The placenta takes over the role of hormone production. The placenta secretes high levels of both oestrogen and progesterone for the remainder of the pregnancy.
FSH is produced in the pituitary gland and it is secreted at the start of the cycle. It stimulates the maturation of a follicle in the ovary and triggers the ovaries to start secreting oestrogen.
Oestrogen is produced in the ovaries (developing follicle). It stimulates the repair and thickening of the uterus lining. High levels of oestrogen inhibit the further release of FSH to prevent multiple eggs from maturing, while stimulating a surge in LH.
LH is produced in the pituitary gland. A sudden surge in LH around Day 14 triggers ovulation and also stimulates the remains of the follicle to develop into the yellow body.
Progesterone is produced in the ovaries (yellow body). It maintains the thickness and blood supply of the uterus lining. It also inhibits the release of both FSH and LH to ensure no new follicles develop during the second half of the cycle.
If fertilization and implantation occur, the hormonal balance shifts to protect the developing embryo.
Lining is maintained. Progesterone levels remain high instead of dropping, preventing the uterus lining from breaking down and also prevents mestruation and losing the embryo.
Uterus and mammary gland development. Oestrogen levels remain high.
Inhibition of FSH and LH. Since progesterone and oestrogen levels remain high, FSH and LH are suppressed to ensure no ovulation occurs during pregnancy.
Placenta handles hormones. The placenta takes over the production of progesterone and oestrogen from the yellow body to maintain the pregnancy until birth.