Pregnancy Probability Statistics 4

What are the odds? The probability of pregnancy is a common concern for both those trying and trying not to conceive, and unfortunately, that’s not a simple question.  Worse, most folks don’t want just a “not likely” or “probably.” They want a number. A statistical chance of pregnancy. Here at Life with Gremlins we have a page explaining the basics of how to get an idea of your chances of pregnancy. We even have a quiz that will give you an answer on a continuum from no chance to high, but in an effort to give you that specific number you’re looking for, now we’re going to delve into the complicated world of conception statistics.

Where to start? Well, how about when you did, intercourse timing.

Chances of Pregnancy by Intercourse timing:

The fertility window is from 5 days prior to ovulation up to ovulation itself. However, it’s important to note this is based on the fact that while most studies agree sperm remain viable up to 5 days (but are most viable in the first 48 hours), research on the egg’s survival range is more conflicting. The mean viability of an egg is generally accepted at 12 hours (the point where in 50 percent of cases, the egg is still viable). However, on the early side, an egg may only survive mere hours and on the late—and longshot— a rare egg may make it a full 48.

The most complete study done on the probability of becoming pregnant on any given day was done in 2013 and published in the Oxford Journal of Human Reproduction. While it’s findings ran similar to others done on the topic, it was the first to counter in maternal age, cycle regularity, and ethnicity. It presented two sets of data, one simply factored in cycle day and ignored ovulation date. The other was based off the chances of pregnancy based on when in the fertile window intercourse was. I know, this seems quite confusing, for example, one graph shows that the maximum chance of pregnancy occurs on day 15 at 13.1 percent, however the other graph shows a maximum chance of 36 percent the day of ovulation.

Why the difference? 

The second, much higher statistic is guaranteeing the sex occurred in a fertile period. So, with data set one, the percentage is skewed by the chance that sex didn’t occur during a fertile time. For example, a woman has a condom break on day 15 of her cycle. There is a 13.1 percent chance that she is both fertile and will become pregnant. Now, assume that she knows she also ovulated that day, now there’s just a 36 percent chance of pregnancy—we already know she’s fertile.

Why am I giving you both?

Data set one is going to be most useful to those who aren’t really trying to conceive and have no idea when they ovulate. The quick and dirty of it is that chances of pregnancy begin close to 0 and then rise around cycle day 7 (with day 1 being the first day of bleeding) reaching a peak at day 15 followed by a slow decline to close to 0 after day 25. This is because most people ovulate around midcycle and the average cycle length is between 28-30 days (hence days 13-16 have the highest chance of pregnancy).
Data set two is going to be more useful to those who are trying to conceive and are entirely sure what day they ovulated.
chances of pregnancy by cycle day chances of pregnancy by days from ovulation

Take away: Considering the high probability of slight miscalculation when predicting ovulation (even with BBT and cycle tracking) most sources average it out to a 20 to 25 percent chance of pregnancy when sex is midcycle.

Timing is far from the only factor in pregnancy probability though, for instance, the chances of pregnancy also change with age.

The Probability of Pregnancy by Age:

In the graph above you see little change between women under 25 and those over 35, but when you look at the percentage of women who became pregnant within a year of trying, the difference becomes clearer. This is a result of declining egg reserve and quality.

chances of pregnancy by age

Take away
: In your late 30’s the chances of pregnancy begin to decline, dropping more dramatically after 40. The vast majority of women under 35 will become pregnant with monthly unprotected sex within one year.

Timing and age are the two primary drivers behind the chances of pregnancy in general (excluding genetics, which is impossible to factor with a wide audience like this. ) Of course, many smaller health issues can play a big role as well. We can’t possibly cover them all, but will try to touch on some of the more common issues.

Factors that can affect your overall chances of pregnancy further:

-BMI: The chances of natural conception reduce 26 percent in those with a BMI over 35, and 46 percent in those with a BMI over 40. Being underweight likewise reduces the chance of pregnancy, with a 37 percent decrease for those with a BMI under 20.

-PCOS: Often holding hands with a high BMI, PCOS is a hormonal condition that usually results in anovulatory cycles and ovarian cysts. Weight gain, facial hair, thinning hair at the top of the head, and irregular periods are the most common symptoms. Around 10 percent women of reproductive age suffer from PCOS, and roughly 80 percent of those women are left infertile.

-Smoking: While research is still conflicting, some studies have found active smokers have a 50 percent lower chance of pregnancy each month.

Activity level: Those who exercise moderately for at least 5 hours a week have an estimated 18 percent higher chance of conception, however, vigorous exercise (more than 5 hours a week, intense workouts) have a roughly 32 percent lower chance of conception. The exception to this was in women who were overweight, in which case fertility often returns with diet, exercise, and weight loss. Less than 5 hours of exercise a week has no effect on fertility.

The male factor:

Obviously, it takes two to become pregnant, and while this website is predominantly visited by women, the other half of the equation can’t be ignored when it comes to pregnancy probability.

Age is the most predominate factor in the chances of pregnancy based on a partner. Men are thought to be most fertile from about age 25 to 29. A 2004 study by the American Journal of Gynecology found that the chances a male will conceive a child within a year of trying drops by 11 percent per year. This means that the older a male partner is, the lower the chance of pregnancy in most cases. Regardless, as a result of age alone, men never cease producing sperm. The oldest known man to father a child was 96 years old.

Other factors on male fertility are less generalized. Things such as environmental exposure to heat or  toxins,  testicular trauma, genetics, and some health conditions can reduce fertility in younger men.

Next we’ll have a look at situational factors in the chances of pregnancy, first with the chances of pregnancy from precum, but as we are already at over 1,200 words, we’ll split that into it’s own page.

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4 thoughts on “Pregnancy Probability Statistics

  • Anonymous

    Hello there!
    My boyfriend (24) and I (20) had unprotected sex 3 times within the fertile window. I’ve been off birth control since March. The past 2 days I have had (TMI ALERT) brownish discharge and its 4 days before my next period. I’ve had nausea, bloating, frequent urination, sore breasts and headaches. Any chances I could be pregnant?? Or could it just be pms?? Thank you!


    Hello, My name is Tracie. I would like to start with the biggest THANK YOU!!!!!!! THANK YOU!!!! for your selflessness. You are a breathe of informational fresh air.

    Lets start with my age. I am 44 years younger and no gremlins. I have only been pregnant twice. I was 18 and 37 both pregnancy ended in miscarriages.

    After a lot of thought and, I mean a lot of thought. My husband and I decided to try to get pregnant. We went to our doctor and was told that all was good and working as it should. I have less eggs of course with me being 44.

    So I started exercising 4 times a week watching what I eat also taking folic acid vitamins along with RX vitamin D I have a deficiency I take 5000 mg once every 7 days. My periods are very regular same time every month.

    June will be 8 months since we started.

    Do you have any suggestions, or words of wisdom.
    I dont track my temperture during my ovulation and i dont chart. I dont want trying to get pregnant consume me or it feels like a job and I miss out on the beginning experience in the ability to create.

    Other then my Doctor saying “Well you are 44 so your changes are slightly lower and it may take longer then expected”.
    I knew that this was going to be a challenge. Im not rushing it. I could with fertility drugs. Im not ready for that option.


    Optimistically Trying.

    • Life with Gremlins admin

      You really sound like you’re doing everything you can already. I get what you mean by not wanting TTC to feel like a chore, you could try some of the less-fuss methods of ovulation tracking such as just paying attention to your discharge (egg-white is often seen at ovulation) or even OPKS. If used just for a few days around mid-cycle to build an average ovulation day, they can be helpful and easy. Of course, just having intercourse every other day or so mid-cycle works too. I wish I could be of more help, but already being healthy with a regular cycle, there isn’t much left beyond fertility meds. If you’re not looking to go that far, you could also try natural fertility boosters such as soy isoflavones, but some find they also can cause cycle upset.