rerun image Although women are grateful for this relatively simple and reliable form of contraception, most of them readily admit that they don't know much about how oral contraceptives actually work.

When you take oral contraceptives, the hormone thins out the lining of the uterus making it unable to host egg implantation. The mucus in the cervix thickens, making it tough for sperm to swim through.

So Many Choices

Most birth control pills contain 2 hormones, estrogen and progestin. Pills that contain both estrogen and progesterone are called combined contraceptives. There is also another type of pill that contains progestin-only, called the mini-pill.

Progestin-Only Pill

Progestin-only birth control pills are a popular choice for many nursing and non-nursing women immediately postpartum. This method is less effective than combination pills. However, it is often recommended after delivering your baby because combination pills increase your risk of developing blood clots.

They may also reduce the risk of triggering migraine headaches compared to pills with estrogen.

Combination Oral Contraceptive Pills

The same kind of estrogen is in all low dose combination pills. The exact dose of estrogen may have an impact on the kind of side effects you experience.

If you have a history of blood clots, or have high blood pressure , your doctor may advise that you take the progestin-only pill. Also, many organizations are concerned that combined oral contraception may suppress milk production, so they do not recommend them in breastfeeding women. Currently, there is not enough evidence to show that birth control pills affect how much milk your body makes.

Monophasic vs. Triphasic

Within the family of estrogen-containing pills, there are 2 main types: monophasic and triphasic. As the names suggest, monophasic pills provide the same level of hormones throughout the pill cycle. Triphasic pills also induce a steady state of hormones, but at 3 different levels during the cycle. The 2 types are equally effective for pregnancy prevention.

Fewer Side Effects Today

There are fewer side effects now than there used to be because oral contraceptives today contain a much lower level of estrogen. Lower hormone levels usually mean fewer side effects, but side effects do still occur.

Negative Side Effects

The most common complaint is about breakthrough bleeding. If you are new to the pill or have recently switched types, you should give your body 3 months to adjust.

Some women believe that they will gain weight when taking the pill, but a study reported that the pill has no major effect on weight.

Other possible side effects include headaches (especially migraines), nausea, breast tenderness, and bloating. If the negative side effects last for more than 3 months, talk to your doctor and consider changing brands.

Positive Side Effects

Many people experience positive side effects. For example, particular types of the pill may improve acne. Also, use of the pill mat decrease a woman’s risk for subsequent ovarian cancer and endometrial cancer.

Your Period on the Pill

Depending on which kind of birth control pill your doctor prescribes, you could have a pack of 21 pills, 28, or even 90.

If there are 21 pills in the pack, take 1 pill at the same time each day for 21 days. After 7 days of not taking a pill, start a new pack. You will have your period during the break.

If your pack has 28 pills, take 1 pill at the same time each day for 28 days. Since the last 7 pills do not contain hormones, this will be when you will have your period.

Another option is to have 28 pills, but with the final 4 pills being inactive (without hormones). With this kind of pill, your period may be shorter and lighter.

Some pills come in packages of 90 pills (also called continuous-dosing or extended-use). The first 83 pills contain hormones, but the last 7 do not. You will have your period during the 7 days with no hormones every 3 months.

There are also packs with 365 pills. Women take 1 pill a day for a year. Over time, bleeding may become lighter and may even stop.

Oops, I Skipped a Day…

Taking a week off after 3 weeks of active hormones is part of a normal pill cycle, but skipping days in the middle of the 3 weeks is not a good idea. It could increase your risk for pregnancy.

If you do miss a pill, take it that day as soon as you remember. Or double up (take 2 pills) if you do not remember until the next day. You should use another method of contraception if you forget to take 2 or more pills. Call your doctor or nurse and ask for advice. Complete directions for what to do are also included in the instructions that come with your birth control pills.

Talk to Your Doctor

You cannot get birth control pills without a prescription, so you have to see a doctor at some point. Be open with your doctor so that you can make the best decision possible about birth control. Women with certain conditions should not take oral contraceptive. Talk to your doctor about any medications you take, as well as any over-the-counter or herbal medications or supplements.

If you experience problems and side effects, speak up. There may be something out there that's better for you.