Since the 1930s, it’s been known that high doses of the female hormones estrogen or progesterone will block ovulation, making conception impossible. This knowledge was employed clinically to create the first birth control pills, consisting of high doses of estrogen and high doses of a progestin. But there were some problems.

High doses of estrogen are associated with an unacceptably high frequency of unpleasant side effects, such as breast tenderness, nausea, headaches, fluid retention and mood changes. High doses were also found to be associated with more serious problems, such as venous thrombosis and pulmonary embolism. Further studies showed that it took only moderate doses of progestin to block ovulation, and high doses of estrogen proved unnecessary. So we have been able to move away from the high dose birth control pills of the 1960s and into the low-dose pills of 1970s and beyond.

When I say low dose, what I mean is a low dose of estrogen. The current low dose birth control pills still employ a medium dose of progestin. Progestin means progesterone-like. Progesterone itself is not used in birth control pills because it doesn’t get absorbed when taken orally. Progestins, on the other hand, are powerful, easily absorbed when taken orally, and usually have very few side effects of their own when taken in moderate doses.

Ideally, the progestin in the birth control pill will block ovulation through suppression of ovarian function. It does this by its’ action on the hypothalamus and pituitary gland, where the hormonal on-off switch to the ovaries is located. By shutting down the release of LH and FSH, the ovaries do not ovulate and their hormonal function is depressed.

Once ovarian function is shut down and ovulation is blocked, pregnancy is prevented. Medium doses of progestin alone can be effective in preventing pregnancy. But with this formulation, woman may notice quite a few indirect effects or symptoms, many of them related to the fact that her ovaries are suppressed and not producing much estrogen any more. Consequently, the pills are usually better tolerated if they contain a little bit (a low dose) of estrogen, to bring her estrogen levels back to close to what they were before she started the birth control pill. Because this level is usually a little lower than what she started with, her periods will be lighter and shorter.

Everything else being equal, the heaviness of a menstrual flow depends on how much estrogen priming takes place before the menstrual period. If there is a lot of estrogen stimulation, the lining will be very thick and the period quite heavy and lengthy. If there isn’t much estrogen stimulation, then the uterine lining will be relatively thin and not bleed much when she gets to her period. If the lining is extremely thin because of very small estrogen stimulation, then the lining becomes somewhat fragile and bleeds easily, a condition recognized by the woman as intermenstrual, unpredictable spotting.

The suppression of ovulation also leads to some other, non-contraceptive benefits, such as reduced menstrual cramps, reduced PMS, reduced breast tenderness, reduced opportunity for functional ovarian cysts to form, reduced risk of ovarian cancer, and suppression of endometriosis. The suppression of ovarian function also leads to a modest decline in testosterone.

You see, I produce both male and female hormone, but mainly male hormone. Women also produce male and female hormone, but mainly female hormone. One-third of the “male hormone” that a woman produces comes from her ovaries, so if her ovaries are suppressed, her serum testosterone level will be somewhat lower. The practical beneficial result of the lowered testosterone is an improvement in her skin with reduction in acne. One uncommon problem with the lowered testosterone level is decreased libido among those women who are very sensitive to this hormone.

Suppression of ovulation is not the only way the birth control pill works. Progestin modifies the uterine cervical mucous, making it impenetrable by sperm, as well as interfering with the normal uterine contraction pattern which squeezes the sperm from the cervix up through the fallopian tubes during ovulation. Progesterone is a smooth muscle relaxant and the uterus exposed to significant amounts of progesterone tends to not contract, but to remain flaccid. Thus, the contraceptive effects of the birth control pill are not limited to just inhibiting ovulation.

That’s a good thing, because the birth control pills are not always perfect in suppressing ovarian function. Different people absorb and metabolize these pills differently, and while the woman will still be reasonably protected against pregnancy, continuing ovulation will lessen the contraceptive effectiveness, and she may not derive the other non-contraceptive benefits of the pill if she is not suppressed.

It is a common experience for me to hear a patient complain that despite taking the birth control pill, her periods are still heavy, her menstrual cramps are worse, and her PMS has not improved. She hasn’t experienced the decrease in acne that we expected. The usual cause for these complaints is that the birth control pill she is taking is not strong enough for her. It is certainly strong enough to protect against pregnancy, but not strong enough to consistently suppress ovulation. This could be for a variety of reasons.

Birth control pills are frequently taken for 21 days, and then stopped for 7 days, to provoke a menstrual flow. For some of the older birth control pills containing progestins with long half-lives, being off the pill for 7 days wasn’t a problem, because it wasn’t long enough to allow ovulation to resume. But for some of the newer progestins with shorter half-lives, taking 7 days off is too long if you want to consistently keep the ovaries suppressed.

Similarly, for women who are a little inconsistent in taking their birth control pills at the same time each day or who skip a day every now and then (which is the majority of women taking birth control pills), the longer half-live pills tend to be more forgiving, while the shorter half-live pills are less forgiving and may allow ovarian function to resume, even though they continue to take the pill.

Some women taking birth control pills will notice that some of their periods are short and pleasant, and other periods long and unpleasant. This means that their birth control pill has effectively suppressed ovulation on some, but not all of her cycles.

Women taking birth control pills who develop two periods each month are also usually on too low a dose of pill. Because their ovulations are not suppressed, they get an ovulation-induced period in the middle of their cycle, as well as the progestin-withdrawal period that they provoke by stopping the pills briefly each month.

You see, aside from the contraceptive benefit, a woman who is not suppressed while taking birth control pills is usually worse off than if she weren’t taking birth control pills at all. She has all of her own hormones, but in addition has all the pill hormones. So rather than the somewhat reduced hormone levels we see in women when the pill is working well, her hormones will be somewhat elevated, contributing to heavy, lengthy flows, menstrual cramps, PMS, breast tenderness, and no improvement in acne.

The solution to these symptoms is usually found by modifying the birth control pill usage in such a way as to make it more effective in suppressing ovarian function. One way is to change the pill frequency, from a 7-day “off week” to a 3-day or 4-day “off week” before resuming pills. Alternatively, you can have the woman take her birth control pills continuously, without any stoppage at all.

Yet another way to resolve this type of problem is to change the pill to one of the older, monphasic pills with long half-live progestins (like levonorgestrel). Whenever you make such a change, you should advise the patient that it may take up to 6 weeks to achieve the ovarian function control you desire, which means that the first cycle on the new pills may not demonstrate an improvement. Instead, look to the second or third cycle on the new pills to see if there is an improvement.

Finally, using a levonorestrel-based monophasic birth control pill continuously will offer the best chance of downregulating or stopping ovarian function and can provide great relief to those women seeking the non-contraceptive benefits so often promised by the pill.

Another application of this knowledge of birth control pill physiology is in counseling patients who are about to start them.

About 80% of women starting birth control pills will have no side effects at all, but the remaining 20% will notice some side effect during the first month they are on the pill. The preparations for ovulation take about 6 weeks, so while it is possible that starting the birth control pill will disrupt an ovulation previously scheduled to occur in two weeks, that might not happen. The reason the side effects go away after the first month is that given the greater length of time, any given birth control pill will be even more likely to suppress ovarian function.

For those women in whom the side effects of headaches, breast tenderness, PMS, heavy flows, and cramps persist past the first month, it is wise to change birth control pills or their dosing to more effectively downregulate ovarian function.