Contraceptive Steroids and their Mechanism of Action
"The Pill," which became a pre-fertilization mode of contraception in the early 1960's, involves a daily dose of estrogen and progestin (an analog to progesterone). Steroid combinations of estrogen and progesterone produce changes in gonadotropin output. (Drill, 1966)
How is the normal hormone cycle altered?
FSH excretion loses its upward-curving character and becomes generally more irregular. The mid-cycle LH surge that initiates ovulation is abolished. Occasionally, a sudden burst of LH activity is seen early on, but no follicles are ripe enough for ovulation at this time. Remember, increased levels of estrodial, which stimulate the hypothalamus to produce high levels of gonadotropins, cause the LH surge. This is why a change in gonadotropin output eventually prevents ovulation. Through experiments involving the direct ovarian response to gonadotropin, results showed that oral contraceptives do not affect the ovarian response, but act to inhibit the release of gonadotropins from the anterior pituitary gland. (Goldzieher, 1966)
(Goldzieher, 1966)
Methods of Administration
What type of regimen therapies are common and available?
1)
The combination type of therapy was first to be distributed. It consists
of a fixed dose of estrogen and progestin for the full twenty days of
the menstrual cycle. (http://pharmacology2000.com/Endocrine/Gonadal/gonad6.htm)
2) The sequential type of therapy was more recently developed in response to the understanding of the "anticonceptive effect." This effect indicated that progestin was largely unnecessary during the beginning of the cycle and was not as effective when in constant combination with estrogen. As a result, this therapy consists of 20-21 days of estrogen administration, with progestin given in addition during the last 5-10 days of estrogen treatment. (Goldzieher, 1966)
3)
Even more recent is the "Mini Pill" which only consists of progestin.
This is an alternative for women who are sensitive to estrogen or cannot
take it due to other reasons, such as breast feeding. The effectiveness
of the progestin-only pill is slightly less effective than that of the
combination type. This occurs because the presence of progestin inhibits
ovulation less consistently than estrogen does. It has an inaccuracy rate
of 3-7%. (http://www.drkoop.com/conditions/ency/article/001946.htm)
(http://www.ultranet.com/~jkimball/BiologyPages/P/Progesterone.html)
What about Progesterone?
Progesterone is primarily responsible for supporting pregnancy through its effects on the endometrium. However, its presence also interferes with the release of FSH and LH, and therefore also aids in the prevention of ovulation. In addition, it may interfere with oocyte transport down the oviduct or with sperm capacitation. This is why it is not produced in the normal cycle until after ovulation. The pill provides progesterone presence early on and ovulation is prevented. (http://www.ultranet.com/~jkimball/BiologyPages/P/Progesterone.html)
(Goldzieher, 1966)
How are the pills administered?
Oral Contraceptive agents are given from day 5 through day 24/25 of the menstrual cycle. If the pill is taken after day nine, virtually no ovulation inhibition occurs. If taken between day 5 and day 9, the pill is virtually guaranteed to be preventive. Ideally, the pill should be consistently taken each day at a particular time. If it is not, there is a possiblity of ineffective prevention. (Goldzieher, 1966)
