Pregnant After Sex
Ever wondered about the chances of getting pregnant after having sex? It's a common question, but the answer isn't a single number—it changes a lot based on a few key factors.
This page has an interactive tool to help you get a personalized estimate. Remember, this is just a rough guide based on scientific data, not a fortune teller! Select the options that best fit your situation to see the result.
Your Personalized Risk Estimate
What birth control method, if any, was used?
How was the method used?
(For example, did the condom break or were any pills missed?)
What is the woman's age?
How many days from the START of your period until the day you had sex?
(Count from the very first day you saw any blood.)
What is your usual menstrual cycle like?
How This Estimate Works (and Its Limits)
This calculator provides an estimate, not a guarantee. It uses a risk model that combines several key factors known to influence the chance of conception. Please keep in mind:
- This is an estimate based on averages from thousands of people. Your individual body and situation may be different.
- The model assumes no underlying fertility issues in either partner (like PCOS or low sperm count).
- Ovulation can vary from cycle to cycle, even for women with regular periods. Stress, illness, and travel can all affect your timing.
Baseline Fertility Parameters
These are single-value constants used in the risk model calculation. The descriptions are linked to the primary sources.
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.
- Reference: Ovulation Calculator, Office on Women's Health, U.S Department of Health & Human Services.
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 |
|---|---|
|
Don't know/unsure |
-1 |
|
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.
- AJ Wilcox, "Timing of Sexual Intercourse in Relation to Ovulation"
Contraception Use Case Logic
This table provides mutually exclusive multipliers to simulate conditional logic. Selecting a row sets one multiplier to 1 and the others to 0.
| logic_description | logic_typical | logic_perfect | logic_failed |
|---|---|---|---|
|
Unsure (Typical Use) |
1 |
0 |
0 |
|
It was used correctly (Perfect Use) |
0 |
1 |
0 |
|
It broke / there was a mistake (Failed Use) |
0 |
0 |
1 |
Contraception Method
This table provides the failure rates for typical, perfect, and failed use cases for each method. A value of 1.0 means no risk reduction.
| bc_description | bc_typical_mult | bc_perfect_mult | bc_failed_mult | bc_n_acts |
|---|---|---|---|---|
|
None (unprotected sex) |
1.0 |
1.0 |
1.0 |
1 |
|
Male Condom |
0.13 |
0.02 |
1.0 |
78 |
|
The Pill (combined) |
0.07 |
0.003 |
1.0 |
78 |
|
Withdrawal ("pulling out") |
0.22 |
0.04 |
1.0 |
78 |
|
Hormonal IUD (Mirena, etc.) |
0.002 |
0.002 |
1.0 |
78 |
|
Copper IUD (Paragard) |
0.008 |
0.008 |
1.0 |
78 |
|
The Implant (Nexplanon) |
0.001 |
0.001 |
1.0 |
78 |
|
The Shot (Depo-Provera) |
0.04 |
0.006 |
1.0 |
78 |
The bc_n_acts column value exists to convert the annual failure rates of birth control provided by the CDC into per act rates, with 78 being if the average person has sex 1-2 times a week (1.5).
- Reference: Contraception, U.S. Centers for Disease Control and Prevention (CDC).
Risk Models
This model uses a Gaussian (bell curve) function to estimate risk.
RiskModel: Pregnant After Sex:PregnancyRiskPerEvent
Content:
{{PregnancyResult
|{{#expr: 0.33*(e^(-0.5*(({timing_intercourse_day_x}-{cycle_ovulation_day_mu})/2.5)^2))*{age_multiplier}*
( (({bc_typical_mult}/{bc_n_acts})*{logic_typical}) +
(({bc_perfect_mult}/{bc_n_acts})*{logic_perfect}) +
({bc_failed_mult}*{logic_failed}) ) }}
|{{#expr: 0.33*{age_multiplier}*
( (({bc_typical_mult}/{bc_n_acts})*{logic_typical}) +
(({bc_perfect_mult}/{bc_n_acts})*{logic_perfect}) +
({bc_failed_mult}*{logic_failed}) ) }}
|{pagestate}
}}
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 and birth control multipliers. This method reflects the rise and fall of fertility throughout the cycle.
If the exact timing isn't known, the maximum fertility adjusted for age (0.33 * age_multiplier) times the birth control effectiveness factor will be shown instead.
The riskmodel logic was getting very complicated and hard to follow, so a template (Template:PregnancyResult) is used, with all of the calculation done here in the riskmodel and the text to display and logic about which text to display done in the template.
Initially created by Gemini (Sept. 2025).