Menstrual Cycle Review

Follicular Phase (1st half of the menstrual cycle)

• This begins with the onset of the menstrual cycle
• Hormone levels (Estrogen, Progesterone and LH) are all at the lowest point of the cycle
• Uterine lining is sloughing off so it can start fresh
• Ovaries do not have any follicles measuring over 12mm
• As ovulation approaches, Estrogen level increases and uterine lining thickens
• One of the ovaries produces a dominant follicle (fluid filled sac that contains an egg). As it grows from less than 10 mm to about 20 mm, the egg inside is maturing, preparing for ovulation

Ovulation

• The follicle reaches maturity, about 20mm in size
• The LH level rises dramatically and briefly
• The Estrogen level tapers off
• The egg inside the follicle matures and loosens from the follicle wall. The follicle itself bursts and the egg is expelled, floating down the fallopian tube where it will survive for approximately 24 hours.

Luteal Phase (2nd half of the menstrual cycle)

• The egg leaves the ovary and travels down the fallopian tube
• The Estrogen level dips down and Progesterone level rises
• Progesterone is vital to the lining of the uterus called the “endometrium.” The lining has been growing thicker with the help of Estrogen; however, the addition of Progesterone is required to produce the right chemical combination for implantation and growth of an embryo
• The endometrial lining must provide the right environment for an embryo to nestle in and begin to grow. Estrogen and Progesterone are the hormones that make this possible. In simple terms, Estrogen makes the endometrial lining “fluffy” while Progesterone makes it “sticky”

Pregnancy

• If an egg meets sperm on the way down the fallopian tube and is successfully fertilized, the newly created embryo will reach the endometrium a few days later and implant
• The corpus luteum (what had been the follicle containing the egg that is now a cyst) continues the production of Progesterone to keep the endometrium thick and healthy
• The newly implanted embryo will begin to produce the pregnancy hormone beta HCG (human chorionic gonadotropin).
• If no fertilization or implantation occurs, the body will stop the production of Progesterone. The rapid decrease in Progesterone levels is what causes another period to begin. The cycle starts all over again with the onset of a period.

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