In women who have regular periods, the risk of pregnancy after a single intercourse is about 5%. This risk is 20 to 30% if intercourse occurs in the middle of the cycle. Checking with a pregnancy test 2 weeks after using emergency contraception is recommended. Most oral emergency contraceptives prevent unwanted pregnancy by preventing or delaying the release of an egg from the ovary (stopping ovulation) using a hormone called levonorgestrel. It is contained in many contraceptive medications. Birth control pills prevent pregnancy by altering the course of the menstrual cycle: The hormones in the pill stop normal ovulation. In essence, stopping the pill returns the body to its natural cycle. Oral contraceptives, like many birth control methods, including patches, injections, rings, and implantable devices, rely on hormones — specifically estrogen and sometimes progestin — to be effective. These forms of contraception work by preventing ovulation and by changing the lining of the uterus and cervical mucus to help block sperm from reaching the egg, fertilizing it and implanting in the uterus. The “active” hormone pills, taken for varying lengths of time and in different doses depending on the brand, are followed by a week of inactive, hormone-free pills—this drop in hormones triggers the uterus to shed the lining, triggering a menstrual cycle. Because low-dose birth control pills are designed to suppress the growth of uterine lining — most of which the body sheds during menstruation — this usually means a lighter, shorter period that’s more regular. Most women’s bodies will return to normal hormone production after stopping birth control pills. In fact, it is possible to get pregnant just a few days after stopping oral contraceptives. On the other hand, it can also take several months for the body to start ovulating and menstruating again on its own schedule – some women will experience a delay before their normal menstrual cycles return, especially those who have had irregular periods before taking oral contraceptives. How to proceed if the menstrual cycle does not normalize? If the menstrual cycle does not normalize after several months, it is necessary to test the levels of hormones, including human chorionic gonadotropin, to check for pregnancy. Testing the levels of other hormones can show if there is a problem with other endocrine glands, such as the pituitary gland. In addition to pregnancy, conditions that can prevent the return of normal menstruation include: Ovarian dysfunction, including premature menopause; High levels of stress; Chronic anxiety; Extreme changes in weight. Any typedose and brand of birth control pills work differently. References: 1. Hubacher D, Lopez L, Steiner MJ, Dorflinger L. Menstrual pattern changes from levonorgestrel subdermal implants and DMPA: systematic review and evidence-based comparisons. Contraception. 2. Milsom I, Lete I, Bjertnaes A, Rokstad K, Lindh I, Gruber CJ, et al. Effects on cycle control and body weight of the combined contraceptive ring, NuvaRing, versus an oral contraceptive containing 30 µg ethinyl estradiol and 3 mg drospirenone. Hum Reprod. 3. Bachmann G, Korner P. Bleeding patterns associated with oral contraceptive use: a review of the literature. Contraception.