Menstrual Disorders - Introduction

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Click the icon to see an image of the pituitary gland.

Ovulation

Ovulation is the process where a mature egg (ovum) is released from the ovary. The egg begins its development inside a follicle of the ovary:

• With the start of each menstrual cycle, follicle-stimulating hormone (FSH) prompts several follicles to mature over a two-week period until their eggs nearly triple in size. Only one follicle becomes dominant, however, during a cycle.
• FSH signals this dominant follicle to produce estrogen, which enters the bloodstream and reaches the uterus. There, estrogen stimulates the cells in the uterine lining to reproduce, therefore thickening the walls.
• Estrogen levels reach their peak around the 14th day of the cycle (counting days beginning with the first day of a period). At that time, they trigger a surge of luteinizing hormone (LH).

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LH serves two important roles:

• First, the LH surge around the 14th cycle day stimulates ovulation. It does this by causing the dominant follicle to burst and release its egg into one of the two fallopian tubes. Once in the fallopian tube, the egg is in place for fertilization.
• Next, LH causes the ruptured follicle to develop into the corpus luteum, a yellow mass of cells. The corpus luteum provides a source of estrogen and progesterone during pregnancy.

Fertilization

The so-called "fertile window" is 6 days long and starts 5 days before ovulation and ends the day of ovulation. Fertilization occurs as follows:

• The sperm can generally survive for up to 5 days once it enters the fallopian tube. The egg survives 12 - 24 hours unless it is fertilized by a sperm.
• If the egg is fertilized, it travels from the fallopian tube into the uterus where it is implanted in the uterine lining and begins its nine-month incubation.
• The placenta forms at the site of the implantation. The placenta is a thick blanket of blood vessels that nourishes the fertilized egg as it develops.
• The corpus luteum (the yellow tissue formed from the ruptured follicle) continues to produce estrogen and progesterone during pregnancy.