Contraception and birth control
The menstrual cycle is all about preparation for pregnancy and depends on a complex interaction between female sex hormones that are produced in peaks and troughs by the ovaries at specific times in the cycle.
Estrogen stimulates egg maturation and levels peak mid-cycle to stimulate ovulation. Progesterone prepares the endometrium (lining of the uterus) for implantation of an embryo. These two hormones are under the control of follicle stimulating hormone (FSH) and luteunising hormone (LH) produced by the pituitary gland, which are in turn under the control of gonadotrophin releasing hormones produced by the hypothalamus in the brain.
Contraception or birth control is based on maintaining consistent levels of estrogen and progesterone, which disrupts normal hormonal interaction to prevent egg maturation and suppress ovulation. Contraception also prevents development of the endometrium, so that if an egg was fertilised it would not be able to implant and grow.
At ovulation cervical mucus changes consistency to become thinner. Contraceptives prevent this change so that the mucus remains thick, forming a physical barrier to sperm.
Types of oral contraceptives
Contraceptive pills contain synthetic hormones that work in the same way as natural estrogens and progesterones. There are four different types of contraceptive pill:
- Combination contraceptive pills that contain a synthetic estrogen and a synthetic progesterone.
- Progesterone alone pills contain a low dose of a synthetic progesterone, and are known as the mini-pill.
- Combination pills that contain a synthetic estrogen and an anti-androgen are used to treat women with symptoms of high androgen levels, while at the same time providing contraception.
- Emergency contraceptive pill contains a high dose of synthetic progesterone and is known as the morning-after pill, as it is a one-off pill taken after unprotected sex.
A synthetic progesterone in an injectable form can also be used for contraception.