What's the Chance of Pregnancy After Unprotected Sex?

Is the probability of winning high?

I had my first sexual intercourse with my boyfriend on November 22 without using a condom.
I took the emergency contraceptive pill, but less than half an hour later, I started vomiting.
Before November 22, there was also some genital rubbing.
My last menstrual period started on November 2, but my cycle is very irregular.
Recently, I have been experiencing vomiting and many symptoms that suggest pregnancy.
What are the chances that I might be pregnant?

Zhen, 10~19 year old female. Ask Date: 2015/11/25

Dr. Lü Lizheng reply Obstetrics and Gynecology

The probability of pregnancy is low; however, if menstruation is delayed, a pregnancy test is still necessary.
Sincerely, Dr.
Li-Cheng Lu, Taoyuan Hospital.

Reply Date: 2015/11/25

More Info

The chance of pregnancy after unprotected sex can vary significantly based on several factors, including the timing of the menstrual cycle, the presence of sperm, and the effectiveness of any emergency contraception taken afterward. In your case, you mentioned having unprotected sex on November 22 and taking emergency contraception shortly after, which is a crucial step in reducing the risk of pregnancy.

Emergency contraceptive pills (ECPs), such as Plan B or Ella, are most effective when taken as soon as possible after unprotected intercourse. They work primarily by delaying ovulation, which means if you take them before ovulation occurs, they can significantly lower the chance of pregnancy. However, if ovulation has already taken place, the effectiveness of ECPs diminishes.
Given that your last menstrual period started on November 2, and considering your cycle is irregular, ovulation could have occurred around mid-November, typically around 14 days after the start of your last period. If you had unprotected sex just before or during ovulation, the risk of pregnancy would be higher. However, since you took emergency contraception within half an hour of the unprotected sex, this would have greatly reduced your chances of becoming pregnant.

The symptoms you are experiencing, such as vomiting and other signs that could indicate pregnancy, can also be attributed to various factors, including hormonal changes, stress, or even side effects from the emergency contraception itself. Nausea is a common side effect of ECPs, and it can occur in some women after taking the medication.

To assess your pregnancy risk more accurately, consider the following:
1. Timing of Intercourse: If you had unprotected sex close to your ovulation window, the risk of pregnancy is higher. However, since you took emergency contraception, this risk is mitigated.

2. Effectiveness of Emergency Contraception: ECPs are about 75-89% effective when taken within 72 hours after unprotected sex. The sooner you take them, the more effective they are.

3. Menstrual Irregularities: Irregular cycles can complicate the timing of ovulation and make it harder to predict your fertile window. If your cycles are typically longer or shorter than average, this can affect when you ovulate.

4. Pregnancy Testing: If your period is late, or if you continue to experience symptoms of pregnancy, it is advisable to take a home pregnancy test. Most tests are reliable about a week after a missed period. If the test is positive, or if you have concerns, consult a healthcare provider for further evaluation.

In conclusion, while there is a chance of pregnancy after unprotected sex, taking emergency contraception significantly reduces that risk, especially when taken promptly. If you are experiencing symptoms that are concerning, it is best to consult with a healthcare professional who can provide personalized advice and support.

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