It is common to wonder if pregnancy can be prevented after a fertile window has passed. This article explains the biological processes involved in conception and outlines methods for preventing pregnancy after an egg has been released but before a pregnancy is fully established.
The Conception Timeline
Pregnancy begins with a precise sequence of biological events. Ovulation occurs when a mature egg is released from an ovary, typically around day 14 of a 28-day menstrual cycle. The egg then travels into the fallopian tube, viable for fertilization for about 12 to 24 hours. Sperm can survive within the female reproductive tract for up to five days, allowing conception from intercourse several days before ovulation.
Fertilization is the union of a sperm and an egg, usually taking place in the fallopian tube. After fertilization, the newly formed single cell, called a zygote, begins to divide and travel towards the uterus. This developing entity becomes a blastocyst by about five to six days after fertilization.
Implantation is the next step, where the blastocyst attaches to the lining of the uterus. This process typically takes place around 6 to 10 days after fertilization. A clinical pregnancy is generally considered to begin after successful implantation.
Emergency Contraception: Preventing Pregnancy Post-Ovulation
Emergency contraception (EC) refers to methods used after unprotected sexual intercourse or contraceptive failure to prevent pregnancy. These methods primarily interfere with the reproductive process before a pregnancy is established.
What Emergency Contraception Is
Emergency contraception reduces the risk of pregnancy after sex without protection. It prevents pregnancy from occurring, rather than terminating an existing one. EC is intended for emergency situations and is not a substitute for routine birth control methods.
Types of Emergency Contraception
There are two main categories of emergency contraception: emergency contraceptive pills (ECPs) and the copper intrauterine device (IUD). Each type has a specific mechanism of action and time sensitivity.
Emergency Contraceptive Pills (ECPs)
ECPs contain hormones that primarily delay or inhibit ovulation, preventing the release of an egg. If ovulation is prevented or delayed, sperm in the reproductive tract will likely die before an egg is released, preventing fertilization. ECPs do not terminate an established pregnancy and are not effective if implantation has already occurred.
Two common types of ECPs are levonorgestrel-based pills (e.g., Plan B) and ulipristal acetate (e.g., ella). Levonorgestrel primarily prevents or delays ovulation by inhibiting the pre-ovulatory luteinizing hormone (LH) surge. It is most effective when taken within 72 hours (three days) of unprotected sex, though it can be used up to 120 hours (five days). Ulipristal acetate works by binding to progesterone receptors, delaying or inhibiting ovulation even when taken shortly before the LH surge. It is effective for up to 120 hours (five days) after unprotected intercourse.
Copper Intrauterine Device (IUD)
The copper IUD is the most effective form of emergency contraception. It is a small, T-shaped device inserted into the uterus by a healthcare professional. The copper IUD prevents pregnancy by creating an inflammatory reaction in the uterus that is toxic to sperm and eggs, preventing fertilization. It also affects the uterine lining, making it unfavorable for implantation.
The copper IUD can be inserted up to 120 hours (five days) after unprotected intercourse. Unlike ECPs, the copper IUD can be effective even if ovulation has already occurred. Once inserted, it can remain in place for many years, providing ongoing contraception.
Time Sensitivity
The effectiveness of emergency contraception methods depends on how soon they are used after unprotected sex. Levonorgestrel ECPs are most effective within 24 hours, with effectiveness decreasing significantly after 72 hours. Ulipristal acetate maintains consistent effectiveness for up to 120 hours. The copper IUD is highly effective throughout its 120-hour window.
Important Considerations and Misconceptions
Efficacy and Side Effects
Emergency contraceptive pills reduce the chance of pregnancy, with ulipristal acetate showing slightly higher effectiveness than levonorgestrel, especially closer to the five-day mark. The copper IUD is over 99% effective as emergency contraception. Common ECP side effects are generally mild and temporary, including headache, nausea, fatigue, dizziness, and irregular vaginal bleeding. If vomiting occurs within a few hours of taking an ECP, another dose may be necessary. Copper IUD side effects can include increased menstrual pain or heavier bleeding.
EC is Not Abortion
Emergency contraception prevents pregnancy and does not cause an abortion. EC methods interfere with ovulation or fertilization, or prevent implantation of a fertilized egg. They do not terminate an existing pregnancy that has already implanted in the uterus.
Not a Regular Contraceptive
Emergency contraception is for occasional, emergency use and should not be a primary method of birth control. Regular birth control methods are more effective at preventing pregnancy over time. Using EC repeatedly exposes the body to higher hormone doses than daily contraceptive pills and does not offer ongoing protection against future acts of unprotected sex.
When to Seek Medical Advice
Consult a healthcare provider if you have concerns about emergency contraception, such as severe abdominal pain or unusual bleeding after taking an ECP. If a period is more than a week late after using EC, a pregnancy test is recommended. Discussing ongoing contraception options with a healthcare professional is also advisable to find a suitable long-term method for preventing pregnancy.