Premenstrual Syndrome - Introduction

(Page 3)

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 survive for up to 3 days once it enters the fallopian tube. The egg survives 12 to 24 hours unless it is fertilized by a sperm.

If the egg is fertilized, about 2 to 4 days later it moves from the fallopian tube into the uterus where it is implanted in the uterine lining and begins its 9-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.

Click the icon to see an image of the placenta.

Click the icon to see an image of the corpus luteum.

If the egg is not fertilized, the corpus luteum degenerates into a form called the corpus albicans, and estrogen and progesterone levels drop. Finally, the endometrial lining sloughs off and is shed during menstruation.

Typical Menstrual Cycle

Menstrual Phases

Typical No. of Days

Hormonal Actions

Follicular (Proliferative) Phase

Cycle Days 1 through 6: Beginning of menstruation to end of blood flow.

Estrogen and progesterone start out at their lowest levels.

FSH levels rise to stimulate maturity of follicles. Ovaries start producing estrogen and levels rise, while progesterone remains low.

Cycle Days 7 - 13: The endometrium (the inner lining of the uterus) thickens to prepare for the egg implantation.

Ovulation

Cycle Day 14:

Surge in LH. Largest follicle bursts and releases egg into fallopian tube.

Luteal (Secretory) Phase, also known as the Premenstrual Phase

Cycle Days 15 - 28:

Ruptured follicle develops into corpus luteum, which produces progesterone. Progesterone and estrogen stimulate blanket of blood vessels to prepare for egg implantation.

If fertilization occurs:

Fertilized egg attaches to blanket of blood vessels that supplies nutrients for the developing placenta. Corpus luteum continues to produce estrogen and progesterone.

If fertilization does not occur:

Corpus luteum deteriorates. Estrogen and progesterone levels drop. The blood vessel lining sloughs off and menstruation begins.