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Given a particular scenario, how can one estimate the odds of becoming pregnant from a single act of intercourse?
In the scenario I propose,
Is there a way to calculate the likelihood of becoming pregnant from such an encounter?
As long the condom don't break you are 99% safe of diseases and babies. Beware that even if the male doesn't ejaculate you can still get pregnant or get diseases if you do not use condoms.
Also you can get pregnant or a disease even if the first time, the second or whatever. Always do safe sex.
All pregnancy tests look for the presence of hCG. hCG is short for "human chorionic gonadotropin" a hormone produced during pregnancy.[1]
To get accurate results, take a pregnancy test 12-14 days after ovulation and retest after you miss your period if you get a negative result. Approximately 90 percent of pregnant women will get a positive result the first day after a missed period, but 10 percent of women will test negative even though they are pregnant.[1]
Your doctor may be able to perform a blood test that can pick up hCG in smaller amounts than a urine test. However, even blood tests cannot tell you whether or not you're pregnant until at least six to eight days after you ovulate, because hCG is not produced until implantation. The soonest you can take a pregnancy test, even if you go to your doctor, is seven days after you ovulate.[1]
There are many facts to consider in answering the question, but in effect, yes, one can calculate the chances. Things to consider:
Condom type
Latex condoms are somewhat more effective than polyurethane condoms, mostly due to breakage and slippage. However, since in your scenario the condom doesn't break (and we assume it didn't slip), breakage or slippage would not be a problem in the calculations. For this scenario, we'll use the effectiveness of a latex condom.
Correct usage
Male condoms remain a highly effective contraceptive method when used correctly. Latex condoms are 98% effective, meaning that two out of 100 women correctly using male condoms as a means of contraception will become pregnant in one year. Use of out-of-date condoms, spermicidal jelly or the wrong kind of lubricant with latex condoms, etc. decreases the efficacy of condoms. In this scenario, we'll assume a not-expired condom, applied properly (not applied and taken off and reapplied, or used more than once, etc.) (Please note, however, it only takes one "successful" mating to become pregnant.)
The menstrual cycle
A menstrual cycle begins on the first day of your period and ends the day before the next period begins. Most women will experience some variation in the length of cycles throughout their lives; normally, though, a cycle is between 21 and 35 days.
Ovulation usually occurs about 14 days before the next cycle begins, but it can vary. If you have a short cycle (say, 24 days) and your bleeding lasts 5 days, this means you likely ovulate around day 10; add one day for every day that your cycle is longer (so in a 35 day cycle, you will likely ovulate on day 21.) Pregnancies occur within a 6 day fertile window. This “fertile window” is comprised of the five days before ovulation and the day of ovulation itself. Sex must occur on one of these days (and, yes, it can be only once. It makes no difference if it's the first time or the 50th.) This is because sperm can live for up to five days, traveling to meet the ovulated egg.
Overall, an estimated 2% of women were in their fertile window by the fourth day of their cycle and 17% by the seventh day (based on 213 women). This percentage peaked on days 12 and 13, when 54% of women were in their fertile window. If ovulation was delayed, women reached their fertile days much later. Among women who reached the fifth week of their cycle, 4-6% were in their fertile window.
This graph shows the probability of women being in their fertile window on a specific day in their cycle.
If a woman has a 28 day cycle (the most common), and stops bleeding on day five, on day 6, there is a less than 20% chance that of her being in her fertile window.
What it boils down to
Even with ejaculation, a latex condom used correctly is between 98% and 99% effective (odds of pregnancy: .02); if the woman has a 28 day cycle, the odds of being in her window of fertility on day 6 is ~15% (.15). Doing the math, .15 x .02 = .0003, which means she has at most a 3 in 10,000 chance of becoming pregnant from this interaction. Because of the lack of ejaculation, the chance is reduced by a factor of 10 (to account for sperm in the pre-ejaculate), so the chance is approximately 3 in 100,000.
That is a very small chance of being pregnant. As a way to put that into perspective, that is only (roughly) 15 times more likely than a person's chance of being killed by an asteroid.
Though the chance is very small, it is not zero. For this reason, one can take a urine test to reassure oneself as soon as the 28th day.
Read the information in the references below to learn how to keep yourself safe while having sex.
Nonlatex vs. latex male condoms for contraception: a systematic review of randomized controlled trials.
Condoms
The timing of the “fertile window” in the menstrual cycle: day specific estimates from a prospective study
