You are a woman of childbearing age.
You had sexual intercourse without contraception.
You do not wish to be pregnant.
What is your risk of getting pregnant?

This is a scenario familiar to many women. Pregnancy can only occur around the time of ovulation, the time when the ovary releases an egg into the fallopian tube. Ovulation occurs midway between menstrual periods if your periods are regular. In medical practice we call the first day of the menstrual period, "day one." The time of ovulation usually occurs midway between first day of two successive periods, but it can vary from day 8 to day 20 of a 28 day cycle.

Figuring the Odds of Being Pregnant

The middle two weeks of your cycle is the time you are most likely to get pregnant. Most women ovulate around day 14, but there is great variation. The time of highest risk for unprotected sex is one or two days before ovulation when there is a 30% chance of becoming pregnant. Three days before ovulation the risk is 15%; on the day of ovulation it is 12%; after ovulation the risk approaches zero.

Women with irregular cycles have more trouble determining when ovulation is occurring. There is a symptom called "mittelschmerz," a term meaning "pain in the middle," that occurs on one side of the abdomen at the time of ovulation. It is caused by spillage of the fluid in the bubble that surrounded the egg when it was on the surface of the ovary and about to be released. That fluid can irritate sensitive surfaces of the abdominal lining and the bowel.

The pain of mittelschmerz is usually deep in the pelvis and can go on for about 12 hours. The monthly pain of mittelschmerz will alternate from one side of the pelvis to the other, because the right and left ovaries take turns ovulating. Not all women experience mittelschmerz, but those who do will have a good idea of when ovulation is occurring in a given cycle. This is true for women with regular or irregular cycles.

So let’s say that you are a woman who gets mittelschmerz, and the day after you had unprotected sex, you felt it. You are at very high risk for getting pregnant—up to 30%.

In the real world, in large studies of women who have unprotected sex in mid-cycle, the risk of getting pregnant is much lower—5-8%. This is because sexual relations on any day other than the 3 days before and the day of ovulation have a very low risk of resulting in a pregnancy.

In the real world, there are other circumstances calling for emergency contraception. One would be an obvious failure of a barrier contraceptive, like a condom with a tear in it. Another is a case of sexual assault, when a woman is raped.

Lowering Your Risk of Pregnancy with Emergency Contraception

So let’s go back to our original line of questioning: What can you do to lower your risk of getting pregnant in these circumstances? You can use emergency contraception (EC). It can decrease the likelihood of getting pregnant by 50-70%. That means that instead of a 5-8% chance of getting pregnant, your chance would be 1-4%. The word "emergency" and the term "the morning after pill" are very apt. You have to get this medication and take it as soon as possible. The sooner you take it, the lower the risk of getting pregnant. The medications described here can be effective up to 5 days after unprotected sex, but their effects decrease with time.

Hormonal Emergency Contraception (EC)
These medications consist of female hormones or derivatives of female hormones that can work in several different ways. The most important effect is a delay in ovulation. Other possible effects are changing the function of the fallopian tubes or altering the lining of the uterus so that a fertilized egg will not implant.

The Copper Intrauterine Device (IUD)
Another effective method of EC is to have an copper IUD placed within 5 days of unprotected sex. This is effective primarily by altering the uterine lining. You need to see a gynecologist to have an IUD inserted.

What Hormonal EC’s Are Available?

Right now there are three FDA-approved preparations available: Next Choice, Plan B One Step, and ella. Next Choice and Plan B One Step are sold over-the-counter to women age 17 and older. They are available for younger women by prescription. Ella is available by prescription only. The cost of these medications is around $40.

Next Choice and Plan B One Step
Next Choice and Plan B One Step contain a type of the hormone progesterone called levonorgestrel. The total dose of levonorgestrel is the same, except with Next Choice you take one 0.75 mg tablet first and a second tablet after 12 hours. With Plan B One Step you just take one 1.5mg tablet. Both preparations should be taken within 72 hours of intercourse. Studies have shown they can work up to 5 days, but become less and less effective as the days go by.

Ella was just approved in 2010. Its active ingredient is ulipristal acetate 30 mg, taken only once. Ulipristal is a selective progesterone receptor modulator, and its mechanisms of action are the same as levonorgestrel. It has a stronger effect on ovulation than levonorgestrel and there is evidence that ella works just as well as the other two medications during the first 72 hours and is more effective for a longer period of time—up to 5 days after intercourse.

Uilpristal belongs to the same class of drugs as mifepristone (Mifeprex), which is currently used in the US for medical abortions. Mifepristone is available in some other countries as an emergency contraceptive in a dose of 10 mg, 1/60 the dose used for medical abortion.

Side Effects and Precautions with EC Drugs

Side effects with progesterone derived EC’s are minimal—headache, menstrual pain, nausea, dizziness, and abdominal pain may occur. Because of the extremely brief exposure to the medication, these effects are short-lived.

Although these drugs are contraindicated in a person known to be pregnant, should they be taken during a pregnancy or should a woman who takes EC become pregnant and decide to continue with her pregnancy, there is no evidence of fetal harm from having used EC.

After Using Emergency Contraception

You took EC. You have reduced your risk of getting pregnant. Now what? You wait, essentially for your next period to come. Under no circumstances should you have unprotected intercourse. Should you decide to have sex before your period, use protection, preferably double protection, the male condom plus the diaphragm, sponge, gel, or foam.

After taking EC and as you are waiting for your period, you may experience certain symptoms. Some women get bleeding or spotting shortly after taking EC. Most women will get their period within three days of the expected date. Women who took levonorgestrel may be a little early; those who took ulipristal may be slightly late. If you have not gotten your period in 21 days, get a pregnancy test and seek medical attention.

Positive pregnancy test or not, this is a great time to see your physician or attend a family planning clinic and have a serious talk about contraception. There are many different choices, depending on your personal situation. Emergency contraception is just that — a good option in an emergency, but it's not a replacement for planned birth control protection.