Community post

Discussion

Any statisticians out there who can help me figure something

Hi there

I am looking for ladies with a good understanding of statistical methods to help me figure something out. I have a decent first year varsity level understanding of statistics. I did it as part of my bachelors degree and again when I did my masters. I did pretty well in the course both times and I feel I have a decent grasp of basic descriptive and inferential statistics, however pregnancy stats has me confused.

As someone who has been ttc for about a year and a half and who has now started the ART journey, I have spent a lot of time on the internet reading various statistics relating to pregnancy and pregnancy rates.
What I do not understand is this:

They say that normal fertile couples have a 15-20% chance of achieving pregnancy every month. However, they also say that around 80% of fertile couples attain pregnancy within a year. (Hence, if you have not gotten pregnant within a year of trying, you are considered infertile and should seek medical assistance.)

Now, if 80% of fertile couples attain pregnancy within a year, it implies that fertile couples have an 80% chance of attaining pregnancy within a year (this is also how one fertility specialist explained it to me - she said that if you are fertile, you have a 20% chance every month, but that this goes up to 80% over the course of a year of active trying).

However, statistically, this does not make sense, as it implies that the 'trials' (each month's trying) are not independent of each other! Surely, they must be! The outcome of one trial cannot possibly have an effect on the outcome of the next. It is like saying that if you flip a coin 10 times and all 10 times you throw heads, the chances of throwing tails on the next throw must be higher than the chance of throwing another heads. But this is not so! It is what is called 'the gambler's fallacy'.
In fact, since the outcome of each flip of the coin is independent of the outcome of the previous flip, the chances of throwing either a heads or a tails remains exactly the same for each flip - i.e. 50%. As the number of flips approaches infinity, you would see the true pattern emerge - i.e. half would be tails and half heads.

So.. back to pregnancy rates: If each month is independent of the last (which it surely must be), then it is completely impossible for the expected monthly pregnancy rate of 15-20% to increase to 80% over a year. It can never be more than 20%, just as the chances of throwing heads in a game of coin toss can never exceed 50%, no matter how many times you flip the coin.

My conclusion then, is that either the stats are a load of hogwash OR the 'trials' are not independent of one another. If the trials are not independent, then what is the connection?

Is there anyone out there who has some insights on this matter?


User Image

4 Replies • 10 years ago


Advertisement

 

Pregnancy test calculator

Use this calculator to help you decide when to take your first home pregnancy test.

1 - 4 of 4 Replies


I failed algebra twice in high school and have literally zero expertise as it relates to statistics. However, I've done plenty of research on becoming pregnant since my husband and I have been trying for over 3 years.

When I read the statistics, I read that 80% of normal, healthy couples will achieve pregnancy within a year. To me, that statement is completely independent and separate from saying that you only have a 20% chance of conceiving each cycle. I felt like you were trying to connect the two, but I might be wrong. :)

The statistics are so low each month(only 20%) because there is literally only a 24 hour window(once a month) in which a woman can become pregnant, technically. 80% of healthy couples will nail this window of opportunity. And I'd say they are mostly accurate, I have become pregnant once each year for the last 3 years-I just can't keep em. (Low hormone levels)

Anyway, I hope this helps? Again, no statistician here, but I have an extremely firm knowledge of how the process works. ;)

10 years ago


Hi there

I appreciate your insight, however, statistics is a branch of mathematics that works in a very particular way.

Doctors are scientists first and foremost, so the statistics that they publish (or give to us) should be open to mathematical scrutiny and I am questioning their stats from a mathematical perspective.

You cannot, statistically (or mathematically), separate the success rate for each month from the overall success rate for the year, unless, like I said, for some reason the trials (each consecutive month's trying) are somehow not independent of one another. Scientifically, I cannot understand how that can be. How can not getting pregnant in month 1 increase my chances of getting pregnant in month 2? It makes no sense.


User Image

10 years ago • Post starter


I was up pretty early this morning and think I figured out the answer to your question. Lets say that 1000 woman trying to conceive are in a study. Each month only 20% of the woman actually conceive, so the first month 200 of the original 1000 conceive and 800 woman progress to month 2. Of those 800, another 20% conceive so they are taken put of the monthly pool and enter the group that conceived. As you progress down the chart the number of women in a given month decreases, while the chance of conception remains the same. When youn get to the end of 12 months, 92.9% of the 1000 women ttc actually succeed. If you factor in miscarriage rates, 80% of the original group of women conceived a viable pregnancy.

10 years ago


Hi there

Yes, you are quite correct. I figured out that what they are doing (in statistical terms) is they are sampling a finite population without replacement (taking 1000 couples and taking the ones that get pg out of the pool, as you put it).
However, this means that their 'trials' are not independent and therefor the observed chance of success increases with each successive trial.

The problem with this is that it gives one a false impression regarding your chances of getting pregnant, because it implies that your chances increase with each passing month.

This is simply not true. Each couple only has a 20% chance of success each month regardless of how many times they try. The point is, yes, most couples achieve pregnancy within a year of trying, but it is a game of chance for everyone and failure in one month does not imply a greater chance of success in the next.

The problem for infertile couples is, our chances are not 20% each month, but far less. Which is why they say that if you do not achieve pregnancy in a year, you should investigate if there is something preventing it.


User Image

10 years ago • Post starter


Log in or sign up to reply to this post.


Early pregnancy symptoms by day past ovulation

What signs and symptoms are most common on each day past ovulation?

 

Advertisement

 

Pregnancy test statistics

Select your day past ovulation to see the statistics and to get an understanding of what result you can expect.

Select your day past ovulation
7
dpo
8
dpo
9
dpo
10
dpo
11
dpo
12
dpo
13
dpo
14
dpo