Contraceptive Methods

Contraceptive Pill

About 10 million women in the United States and 60 million women worldwide are currently using oral contraceptives. These contraceptives usually consist of a synthetic estrogen combined with a synthetic progesterone in the form of pills that

Decreased negative feedback inhibition

1 Estradiol;! Progesterone [Day 28]

Corpus luteum regresses

Ovaries

Anterior pituitary t

I GnRH

t Estradiol;! Progesterone

Empty follicle becomes a corpus luteum t Estradiol

Hypothalamus

GnRH

Anterior pituitary

FSH and LH

Ovaries t Estradiol

Hypothalamus

Negative feedback

Increased sensitivity of follicles to FSH

Growth of follicles

t Estradiol

Positive feedback

Anterior pituitary

LH surge

Ovaries

Ovulation

■ Figure 20.37 Endocrine control of the ovarian cycle. This sequence of events is shown together with the associated phases of the endometrium during the menstrual cycle.

are taken once each day for 3 weeks after the last day of a menstrual period. This procedure causes an immediate increase in blood levels of ovarian steroids (from the pill), which is maintained for the normal duration of a monthly cycle. As a result of negative feedback inhibition of gonadotropin secretion, ovulation never occurs. The entire cycle is like a false luteal phase, with high levels of progesterone and estrogen and low levels of gonadotropins.

Since the contraceptive pills contain ovarian steroid hormones, the endometrium proliferates and becomes secretory just as it does during a normal cycle. In order to prevent an abnormal growth of the endometrium, women stop taking the steroid pills after 3 weeks (placebo pills are taken during the fourth week).

1 Fox: Human Physiology, Eighth Edition

1 20. Reproduction

1 Text

© The McGraw-Hill Companies, 2003

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Chapter Twenty

Table 20.6 Phases of the Menstrual Cycle

Phase of Cycle

Hormonal Changes

Tissue Changes

Ovarian Endometrial

Pituitary

Ovary

Ovarian

Endometrial

Follicular (days 1-4) Menstrual

FSH and LH secretion low

Estradiol and progesterone remain low

Primary follicles grow

Outer two-thirds of endometrium is shed with accompanying bleeding

Follicular (days 5-13) Proliferative

FSH slightly higher than LH secretion in early follicular phase

Estradiol secretion rises (due to FSH stimulation of follicles)

Follicles grow; graafian follicle develops (due to FSH stimulation)

Mitotic division increases thickness of endometrium; spiral arteries develop (due to estradiol stimulation)

Ovulatory (day 14) Proliferative Luteal (days 15-28) Secretory

LH surge (and increased FSH) stimulated by positive feedback from estradiol LH and FSH decrease (due to negative feedback from steroids)

Estradiol secretion falls

Progesterone and estrogen secretion increase, then fall

Graafian follicle ruptures and secondary oocyte is extruded into uterine tube

Development of corpus luteum (due to LH stimulation); regression of corpus luteum

No change

Glandular development in in endometrium (due to progesterone stimulation)

This causes estrogen and progesterone levels to fall, permitting menstruation to occur.

The side effects of earlier versions of the birth control pill have been reduced through a decrease in the content of estrogen and through the use of newer generations of progestogens (analogues of progesterone). The newer contraceptive pills are very effective and have a number of beneficial side effects, including a reduced risk for endometrial and ovarian cancer, and a reduction in osteoporosis. However, there may be an increased risk for breast cancer, and possibly cervical cancer, with oral contraceptives.

Newer systems for delivery of contraceptive steroids are designed so that the steroids are not taken orally, and as a result do not have to pass through the liver before entering the general circulation. (All drugs taken orally pass from the hepatic portal vein to the liver before they are delivered to any other organ, as described in chapter 18.) This permits lower doses of hormones to be effective. Such systems include a subcutaneous implant (Norplant), which need only be replaced after 5 years, and vaginal rings, which can be worn for three weeks. The long-term safety of these newer methods has not yet been established.

Rhythm Method

Studies have demonstrated that the likelihood of a pregnancy is close to zero if coitus occurs more than 6 days prior to ovulation, and that the likelihood is very low if coitus occurs more than a day following ovulation. Conception is most likely to result when intercourse takes place 1 to 2 days prior to ovulation.

There is no evidence for differences in the sex ratio of babies conceived at these different times.

Cyclic changes in ovarian hormone secretion also cause cyclic changes in basal body temperature. In the rhythm method of birth control, a woman measures her oral basal body temperature upon waking to determine when ovulation has occurred. On the day of the LH peak, when estradiol secretion begins to decline, there is a slight drop in basal body temperature. Starting about 1 day after the LH peak, the basal body temperature sharply rises as a result of progesterone secretion, and it remains elevated throughout the luteal phase of the cycle (fig. 20.38). The day of ovulation for that month's cycle can be accurately determined by this method, making the method useful if conception is desired. Since the day of the cycle on which ovulation occurs is quite variable in many women, however, the rhythm method is not very reliable for contraception by predicting when the next ovulation will occur. The contraceptive pill is a statistically more effective means of birth control.

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