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Women who have decided to go on the pill generally hope that it will start being effective right away, but it's not quite that simple.

There are some rules to follow if you want to ensure that you do not become pregnant.

Combined oral contraceptives are very effective if they are taken at the same time every day and you follow all other instructions to the letter. With typical use, the birth control pill is 92 percent effective, which means eight in 100 women get pregnant every year.

With perfect use, your contraceptive can be 99 percent effective, so make sure you use it correctly.

Your healthcare provider will give you instructions on how to commence pill use. There are a few different methods to ease into pill use.

Some women start taking the birth control pill on the first day of their period. In this case, the pill starts working right away and you do not need to use an additional form of contraceptive as a back-up (though you can if that makes you feel more comfortable).

Others use the "first Sunday after period" method to start the pill. With this method, you start the first pill in the pack on the first Sunday after your menstrual period. You will need to use condoms for seven days after commencing, and the birth control pill will then be effective.

The third alternative is to start using the birth control pill right away, regardless of where in your menstrual cycle you currently are. In this case, you will also need to use a back-up method of contraception for seven days before your combined oral contraceptives start working. Ensure that you follow all other instructions closely.

The oral contraceptive pill has four main mechanisms of action:

  • suppression of ovulation,
  • altering cervical mucus,
  • interfering with endometrial proliferation
  • interfering with ovum transport.
The female menstrual cycle is controlled by hormones emitted from the hypothalamus, pituitary gland and ovaries.

The hypothalamus secretes gonadotropin-releasing hormone (GnRH), which triggers the pituitary gland to release follicle-stimulating hormone (FSH) and luteinizing hormone. FSH signals the granulosa cells in the developing follicle to secrete estrogen and promotes follicular development.

A high dose of estrogen taken during the first 10 days of the cycle will stop maturation of the follicle, and prevent ovulation. The progesterone in the pill exerts a negative feedback on the hypothalamus reducing the pulsatile secretion of gonadotropin-releasing hormone. This, paired with the absence of estrogen-positive feedback, prevents the mid-cycle surge in luteinizing hormone, again working to prevent ovulation.

Estrogen creates "hostile cervical mucus" that prevents sperm from reaching the egg -- most will never get beyond the vagina. It also inhibits the contractions of the fallopian tubes that would otherwise aid the transportation of egg and sperm.

Progesterone is needed to "ripen" the endometrium -- the lining of the uterus -- and make it ready for implantation. A lack of ideal plasma concentrations of progesterone and estrogen causes a degeneration of the endometrium, and reduces water and electrolyte loss. These actions mean that implantation is close to impossible, even in the unlikely event that an egg was fertilized.

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