Monday, November 28, 2011

The Science behind Birth Control Pills

Birth control pills are the most widely used non-permanent method of contraception. A recent study found that nearly one-third of U.S. women practicing contraception are currently using hormonal birth control pills. However, the scientific mechanics behind the pill are largely ignored in sexual education classes and rarely explained by doctors. Many women find themselves daily taking a drug they know very little about.

At its core, hormonal birth control (HBC) uses synthetic hormones to regulate the monthly cycle. There are two types of birth control pills: The combination pill, which contains both estrogen and progestin (a class of progesterone) and the progestin-only pill. Both types suppress ovulation and prevent implantation of a fertilized ovum in the uterus, but differ in how this occurs.

The combination pill inhibits production of Gonadotropin-releasing hormone (GnRH). GnRH releases two hormones from the anterior pituitary essential to the monthly cycle: Follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These two hormones are required for ovulation (the release of the ovum or egg from the ovary) to take place. Without them no ovum matures, no ovulation takes place, and no pregnancy can begin.

Because the combination pill stops these hormones at their source, it is extremely effective at preventing ovulation. However, the combination pill has other contraceptive effects that become very important when using emergency contraception. Synthetic estrogen speeds the rate of ovum transport through the oviduct, decreasing the window of time fertilization can occur. Progestin stimulates the production of thick, pasty cervical mucus that makes sperm travel difficult. It also prevents adequate growth of the endometrium (layer of tissue along the uterine wall), creating a hostile environment for implantation.

Progestin-only pills, often called the mini pill, inhibit LH and FSH levels at the anterior pituitary. These pills are just slightly less effective than the combination pill at preventing ovulation. Without LH and FSH, no ovum matures and no ovulation takes place. This method also has a number of secondary contraceptive effects. Like the combination pill, progestin thickens the consistency of cervical mucus and decreases endometrial growth. However, progestin has an opposite effect on ovum transport. Slowing down the ovum’s movement through the oviduct means the endometrium will begin to break down before it arrives, further preventing implantation.

Mini pills are a slightly less effective contraception option than combination pills. Chances of getting pregnant on the combination pill with perfect use are 0.1% versus 0.5% for the mini pill.  The chances of getting pregnant on both the combination pill and mini pill with standard use are around 5%. The drastic drop in effectiveness with typical use occurs when women do not take their birth control pill at the same time every day. Blood levels increase as the synthetic hormones are absorbed into the bloodstream and then decrease over the next 24 hours. It is very important to take the next pill before blood levels return to normal and natural hormone production begins.

Both birth control pills decrease the risk of endometrial, colorectal, and ovarian cancers. Recent studies find women are still at a lower risk for these cancers years after they discontinue use. A woman on HBC will have reduced menstrual flow because of reduced growth of the endometrium. This also results in reduced production of prostaglandins. Fewer prostaglandins mean decreased contractions of the uterus during menses, lessening the severity of dysmenorrhea (menstrual cramps).

There are risks associated with both the combination pill and mini pill. The estrogen in the combination pill increases the risk of blood clots, stroke and heart attack. These risks are higher for women who smoke, are over 35 years old, have a family history of heart attack, or are already at risk for blood clots. There also seems to be a correlation between breast cancer and estrogen, though results are inconclusive and more testing is needed. Mini pills increase the risk of high cholesterol levels and cardiovascular disease caused by plasma lipids. Yaz, a specific brand of progestin-only pills, contains a new form of progestin called drospirenone, and has additional safety concerns. These pills raise potassium levels and can lead to kidney, liver, and heart problems. Both pills can cause irregular menses or amenorrhea, the absence of menses, and do not protect against STIs. Every woman should consult with her doctor to find the birth control pill that best fits with her medical needs.