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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. The corpus luteum provides a source of estrogen and progesterone during pregnancy.
Fertilization. The so-called "fertile window" is six days long and starts five days before ovulation and ends the day of ovulation. Fertilization occurs as follows:
- The sperm can survive for up to three 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 two to four days later it moves 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.
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Click the icon to see an image of the placenta. |
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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
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Menstrual Phases
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Typical No. of Days
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Hormonal Actions
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Follicular (Proliferative) Phase
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Cycle Days 1 through 6: Beginning of menstruation to end of blood flow.
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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.
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