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How do Oral Contraceptives Prevent Pregnancy? Their Mechanism of Action

mechanism of action of contraceptives
Oral contraceptives prevent conception/pregnancy due to their following effects on a woman’s body.
1. Oral contraceptive pills contain Estrogen and a Progestogen. The most commonly used Estrogen in oral contraceptive pills is  Ethinyl Estradiol while the commonly use Progestogens are Norethindrone, Levonorgestrel, Desogestrel and Norgestimate. You can see their names written in the leaflet that is supplied along with the pill pack. 
2. Oral contraceptives inhibit ovulation. they inhibit the release of the egg from the ovaries and without the egg, the male sperm can’t produce a baby. Oral contraceptives inhibit ovulation by manipulating the normal hormonal responses and blood level of a female. Hypothalamus secretes a hormone called Gonadotropin-releasing hormone (GnRH). This acts on the anterior pituitary and causes the release of FSH and LH. FSH reaches the ovaries via blood where it causes follicle maturation and development. It also signals the release of Estrogen from the follicle. Estrogen starts increasing however this increase does not occur all of a sudden rather Estrogen increases steadily. A gradual increase in the Estrogen for about 70 hours signals the LH surge. In LH surge, you have a sudden increase in the LH level and this LH surge is what will bring about the subsequent ovulation. Keep in mind that this gradual increase in the estrogen is very important for the LH surge and ovulation to occur and this is what is interfered by combined oral contraceptive pills.
Basically, oral contraceptive pills will keep the levels of Estrogen constantly elevated. When Estrogen levels remain constantly elevated there will be no LH surge. If LH surge doesn’t occur there will be no ovulation and there will be no eggs and no pregnancy will occur. This is how contraceptives prevent a pregnancy to occur.
3. They make the cervical mucus thick so that sperms cannot go up to the uterus and tubes easily.
4. These pills also make the uterine wall unfavourable for the attachment of embryo.

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