Lies, Damn Lies, and Pregnancy Statistics
June 17, 2015 7:21 AM   Subscribe

There is a statistic that gets thrown at all women trying to get pregnant stating that each month that you only have a 20% chance of getting pregnant assuming each partner is fertile and otherwise healthy. This seems super low (and depressing and discouraging) to me. What gives?

In lots of digging, I found what I think(?) is the study that is the basis for the statistic, but I am having issues figuring out exactly where it's getting this 20% stat. But in looking at the study method, while they tracked BBT and fertility windows, they didn't seem to only look at couples having unprotected sex during the fertile windows, just at subjects who became pregnant during the study.

Am I totally misreading this paper?

Smart, academic based MeFites - can you help discern how this paper gets cited as the 20% change of getting pregnant? Or do you know of a more concrete study that shows this?
posted by Suffocating Kitty to Health & Fitness (19 answers total) 6 users marked this as a favorite
According to Expecting Better, the odds are closer to 50% (depending on age and other factors), assuming the couple has sex on the correct small number of days. If data about pregnancy is what you crave, I recommend that book.
posted by Phredward at 7:26 AM on June 17, 2015 [6 favorites]

I notice they identified the fertile window with basal temp and mucus observations and I wonder whether that could have skewed the results low.

Maybe you've already done this, but if you haven't, I'd suggest buying a several-month supply of real ovulation predictor strips (I think I got mine from which will give you an actual, no-errors-possible read on when you're ovulating. I say this because when I used the strips I got the surprise of my life as to when my ovulation was happening. If I had used the conventional wisdom on fertile windows I'd still be waiting for my kids to happen. Good luck.
posted by fingersandtoes at 7:49 AM on June 17, 2015 [4 favorites]

Are you considering that something like 50% of fertilized eggs miscarry so early that the woman doesn't know she's pregnant? They're probably counting that as part of the "not pregnant" 80%.
posted by Jacqueline at 7:53 AM on June 17, 2015 [6 favorites]

A 20% chance of getting pregnant each month works out to a 93% chance of getting pregnant within a year, which sounds pretty high to me.
posted by yarntheory at 7:55 AM on June 17, 2015 [15 favorites]

I haven't heard that particular statistic. I wonder if it applies to each time you have sex during the fertile period (I.e. more sex during that window, higher probability of getting pregnant?)

If you really want to depress yourself, look at the success rates cited by IVF doctors (I think the success rates quoted to me for my last cycle, based on my age and history, we're something like 5%. But here I sit nursing a 2wk-old).

My RE also told me something interesting, which I had never heard before. All my life I've heard that "one egg ripens at just the right time..." but the doc told me that actually our bodies are producing many mature eggs every single day. They are just waiting for that right chemical combination of hormones for the one lucky egg to fully mature and break through for fertilization.

So the stats are okay for trying to explain things in abstract, but every body is different as they say.
posted by vignettist at 8:09 AM on June 17, 2015

Best answer: I don't think that study is the source of that statistic. How did you determine that as the source? It just doesn't look like a major paper - Italian paper from the year 2000 in an unknown journal?

There's a pretty good breakdown on that stat and related stats, such as the one noted by yarn theory, here. That uses a 1956 paper by a well known researcher, Alan Guttmacher (of Guttmacher Institute fame) published in JAMA, one of the most prominent medical journals in the world. Trying to use my library access to view it but no luck right at the moment - if you want to read it I can get it for you later today.

Regarding stats quoted by infertility docs noted above, your personal stats look completely different after you've been having unprotected sex for at least 6 months to a year without success. That doesn't apply to the general population who has normal fertility.
posted by treehorn+bunny at 8:13 AM on June 17, 2015 [2 favorites]

Best answer: I'd also like to quote this much more encouraging, but also rather funny passage from the beginning of the Guttmacher paper I linked to above:

"The length of time for a first conception to occur is calculated from data from six geographical areas; the median lies between two and three months. The previous use of contraceptives does not lengthen the time required for conception in women achieving pregnancy. The optimum coital frequency for conception is found to be four times or more weekly. The decline of coital frequency with age is but one factor in the strong antifertile influence of length of marriage."
posted by treehorn+bunny at 8:17 AM on June 17, 2015 [3 favorites]

A 20% chance of getting pregnant each month works out to a 93% chance of getting pregnant within a year, which sounds pretty high to me.

My wife and I are in fertility counseling at the moment, and our doctors have quoted the 20% statistic per month, and that 85% of people will become pregnant within their first year of trying, so that's not too far off.

After the first year, it drops to 15% a month. I'm not sure if their stats are coming from the same study referenced in the question or from their own practical experience.
posted by LionIndex at 8:19 AM on June 17, 2015 [1 favorite]

Miscarriage is extremely common, especially very early ones in the first few weeks, doctors point to as high as 33%. As others have noted if those are being included in the calculation 20% per month sounds totally reasonable.
posted by French Fry at 8:48 AM on June 17, 2015 [2 favorites]

In case it's not clear, all of the stats listed above are reasonably consistent with each other and also with what you said. Most people are detectably pregnant after 6-12 months of trying, about half of them after about 3 months. It just about works out to be a dice roll once per menstrual cycle, with slightly better stats under certain conditions. Another question might be: what part of those stats are depressing to you? That may be a stats piece you're missing, hard to tell.
posted by tchemgrrl at 9:28 AM on June 17, 2015

Response by poster: treehorn+bunny: In searching for the 20% change state, I came across a BBC news articles that linked to the above study when they quoted the stat. If you wouldn't mind sending me the paper, I would GREATLY appreciate it. (but please don't go out of your way).

It's a little depressing to think that even if you time everything right, you know your ovulation day, you are having sex before and after, that you still have a very low chance of getting pregnant each month. (or maybe you don't, maybe the stat only means over the course of a month of unprotected sex as opposed to planned sex during ovulation - that's what I was looking to find out).

Follow up question: If you have a 20% chance of getting pregnant each month, how does that percentage go up over the course of a year? Wouldn't it be treated as a new coin toss every month without additive odds? (i am NOT a mathematician by any stretch, so if this is an entry level stats question - please pardon my lack of knowledge)
posted by Suffocating Kitty at 10:54 AM on June 17, 2015

I can't do the math for you*, but intuitively: If you flip a coin, you have a 50% chance of getting heads. So it's not so surprising if you flip a coin once and get heads. But what if you flip a coin 10 times and get heads every time -- that's a surprising and unlikely result. So it's less of "additive" odds, and more of "subtractive" odds, where the chances of getting a given outcome many times in a row (in your case, lack of conception) get smaller and smaller the more times you try.

It's important to remember though, that any *given* month, you still only have the 20% chance (it's not like just because you've gotten heads 9 times, you now have a 90% chance to get tails the next time).

*For the math: Look under the section titled " Two or More Events" here.
posted by sparklemotion at 11:07 AM on June 17, 2015

Although a woman can get pregnant any time in the cycle, a woman is much more likely to get pregnant during the ovulation window. Any study that just looks at people banging around during the whole of the menstrual cycle will not reflect what would happen if a couple had coitus a concentrated number of times during the ovulation window.

In regards to your math question it works out this way:

80% not pregnant first month, 20% pregnant.

Of those 80% not pregnant, 20% of these will be pregnant the second month which works out to be 16%. (0.8 X 0.2, i.e., 16% second month plus 20% first month equals 36%).

Of those 64% not pregnant after two months (100% - 36%), 20% will get pregnant, or 12.8% (0.64 X 0.20). So that is 20% (1st month) + 16% (2nd month) + 12.8% (3rd month) or 48.8% after three months.

You can continue on this calculation figuring 20% of the remainder each month. This assumes these are healthy individuals and there is nothing preventing the pregnancy.
posted by dances_with_sneetches at 11:10 AM on June 17, 2015 [4 favorites]

this advice is probably unsolicited so i apologize. try not to get too wrapped around the axle about statistics. i was upset about that 20% too. i am 39. I got pregnant one month off of BC the first time trying at 37. And i'm now preggars again after trying for two months. I defied statistics. so might you.
posted by SanSebastien at 12:33 PM on June 17, 2015

Best answer: When I was trying to conceive I read The Impatient Woman’s Guide to Getting Pregnant by Jean Twenge. She breaks down those often quoted stats early on (with citations). She talks about one study where couples had sex twice a week, and on average a woman under 35 would be pregnant in 3 months, and a woman under 40 would be pregnant in 4 months. There’s also a study of women who were practicing fertility awareness; better 60% of women under 35 conceived the first month they tried. Of course, there’s still the miscarriage stats to subtract, but even so.

If you’re interested in trying fertility awareness, Taking Charge of Your Fertility by Toni Weschler is invaluable. For that matter, I wish I’d read it years before I was trying to get pregnant. It gave me a much better appreciation for my own body.

As a bit of anecdata: I got pregnant twice in my very late 30s, each on our first cycle of trying, using that method (one miscarriage, one successful pregnancy).
posted by Kriesa at 1:36 PM on June 17, 2015 [1 favorite]


how does that percentage go up over the course of a year? Wouldn't it be treated as a new coin toss every month without additive odds?

Nope, as dances with sneetches points out, although the odds of getting pregnant each month don't get better, the odds that you will have gotten pregnant DO get better over time - subtle but important difference! the first link I posted above answers that question with this table:
Month Monthly Probability Cumulative Probability
1 0.2 0.20
2 0.2 0.36
3 0.2 0.49
4 0.2 0.59
5 0.2 0.67
6 0.2 0.74
7 0.2 0.79
8 0.2 0.83
9 0.2 0.86
10 0.2 0.89
11 0.2 0.91
12 0.2 0.93

so, as you can see, the majority of the women in this group will be pregnant after about 3-4 months of trying. It's really not that long. Take it from a woman who took 2 years to get pregnant with her first child, and couldn't do it unassisted...

I'll go get you that paper now! Just MeMail me your e-mail that I can send it to. The BBC article was probably not written by someone with a science background - they probably didn't realize that same statistic is probably quoted in hundreds of other papers but you have to follow the citations to go back to the original source.
posted by treehorn+bunny at 6:50 PM on June 17, 2015 [2 favorites]

If it encourages you at all, four months of unprotected sex is what it took for me to get a woman pregnant who had been told by a doctor that she had a 0% chance of getting pregnant.
posted by clawsoon at 11:41 PM on June 17, 2015 [1 favorite]

I was really looking forward to "trying to have a baby". Took less than a month. I would have really enjoyed a few months of trying.

By the way, miscarriages are really common especially at the very beginning. A lot of "late periods" can actually be miscarriages. Don't be discouraged if you have an early miscarriage. Unless your doctor says otherwise, you can try again right away and can still get pregnant easily.
posted by Deathalicious at 1:40 AM on June 18, 2015 [1 favorite]

Amen to everything treehorn+bunny said, especially this: "the majority of the women in this group will be pregnant after about 3-4 months of trying. It's really not that long."

FWIW, this matches up with my own anecdata: at age 30 and 32, I got pregnant each time on the first cycle, and now have 2 healthy children. At age 38, I tried for 4 cycles and had one very early miscarriage (technically termed a "chemical pregnancy") before deciding I did not want to have any more children. My friend from my prior Ask, who is now age 38, is due to deliver her first baby in August, after so many heartbreaking losses and almost 5 years of trying. YMMV, but statistically speaking, you have every reason to be optimistic. Seconding the excellent book Taking Charge of Your Fertility.
posted by hush at 1:46 PM on June 18, 2015

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