Chances of Getting Pregnant After Sex/Data
This subpage contains the data and risk models used on the main page. The data is based on population averages and is used to generate a risk estimate.
Baseline Fertility Parameters
These are single-value constants used in the risk model calculation. The descriptions are linked to the primary sources.
- Peak Conception Probability (A): 0.33
- Fertile Window Width (σ, in days): 2.5
- Approximation of 'e': 2.71828
Age-Related Fertility
This table provides a final multiplier based on the woman's age, which adjusts the overall result from the bell curve calculation.
| age_description | age_multiplier |
|---|---|
|
Under 25 |
1.0 |
|
25-30 |
0.9 |
|
31-39 |
0.65 |
|
40 and over |
0.25 |
- Reference: Age and Fertility, American Society for Reproductive Medicine (ASRM), 2021.
Menstrual Cycle Profile
This table provides the estimated peak ovulation day (μ) for different cycle profiles. A value of -1 indicates an irregular cycle where a peak cannot be reliably predicted.
| cycle_description | cycle_ovulation_day_mu |
|---|---|
|
Regular and short (25 days or less) |
11 |
|
Regular and average (26-31 days) |
14 |
|
Regular and long (32 days or more) |
18 |
|
Irregular or I don't know |
-1 |
- Reference: Your Menstrual Cycle, Office on Women's Health, U.S. Department of Health & Human Services, 2021.
Timing of Intercourse
This table converts the user's time window into a single day of the cycle (x) for use in the calculation (using the midpoint of the range).
| timing_description | timing_intercourse_day_x |
|---|---|
|
1-7 days (During or just after my period) |
4 |
|
8-13 days (About a week after my period) |
11 |
|
14-20 days (About two weeks after my period) |
17 |
|
21-28 days (In the week my next period was due) |
25 |
|
More than 28 days |
-1 |
- Reference: Based on standard menstrual cycle phases.
Contraception Method
This table provides the typical use failure rate for common birth control methods, which acts as a risk multiplier. A value of 1.0 means no risk reduction.
| bc_description | bc_failure_multiplier |
|---|---|
|
None (unprotected sex) |
1.0 |
|
Male Condom |
0.13 |
|
The Pill (combined) |
0.07 |
|
Withdrawal ("pulling out") |
0.22 |
|
Hormonal IUD (Mirena, etc.) |
0.002 |
|
Copper IUD (Paragard) |
0.008 |
|
The Implant (Nexplanon) |
0.001 |
|
The Shot (Depo-Provera) |
0.04 |
- Reference: [1], U.S. Centers for Disease Control and Prevention (CDC).
Risk Models
This model uses a Gaussian (bell curve) function to estimate risk, approximating the mathematical constant 'e' with 2.71828.
RiskModel: Chances of Getting Pregnant After Sex/Data:PregnancyRiskPerEvent
Content:
'''Result:''' {{#ifexpr: {cycle_ovulation_day_mu} < 0 or {timing_intercourse_day_x} < 0
| ''Because your cycle is irregular or the timing is too uncertain, it's not possible to provide a reliable estimate with this tool.''
| Your estimated chance of pregnancy is about '''{{One_In_X|{result}}}''' from this event.
}}
{{#ifexpr: {result} > 0.01
|== What You Can Do Now ==
If you have had unprotected sex recently and do not wish to become pregnant, you have options. It's important to act quickly.
* '''Emergency Contraception (EC):''' Often called the "morning-after pill," EC can prevent pregnancy if taken soon after sex. It is most effective the sooner you take it, and can be used up to '''3-5 days''' after unprotected sex, depending on the type. You can often get EC from a pharmacy without a prescription.
* '''Talk to a Professional:''' A pharmacist, doctor, or a local health clinic (like a school nurse or Planned Parenthood) can provide confidential advice, EC, and other contraceptive options.
}}
Calculation Explanation: 🔢 This model calculates the distance in days between intercourse (`x`) and the estimated day of ovulation (`μ`), then inputs that distance into a bell curve formula to find the corresponding probability. This result is then adjusted by the age_multiplier. This method more accurately reflects the rise and fall of fertility throughout the cycle.
Initially created by Gemini (Sept. 2025).