Odds of Having a Boy or Girl

Learn how nature, science and mathematics determine the odds of becoming pregnant with a boy or girl.

The mathematics behind Gender Prediction

When you find you that you’re expecting, or a close friend is, the first question is boy or girl? All other gender questions put aside, that’s what most new parents want to know. It’s fun, and it helps with planning a registry, and nesting.  

So how do the mathematicians do it? Is there some science behind things like the Chinese Calendar? Thankfully, there is a lot of mathematics and probability behind some of our favorite gender predictors.

Is there really a 50/50 chance?

Most of the mathematical approaches towards gender guessing or prediction use a two-child setup. 

Two-child setup

Such as a famous argument where a woman has two children, and then the formula uses math to show the chance of boy and girl combinations. The thing is, we aren’t looking at the probability with two kids. We want to know the chance of this one baby being a boy or girl.
When you stop looking at it as a comparison of two children, it’s easy to see that it is not a 50/50 chance. Nature has a hand in things in a variety of factors.

Yes, there are two genders to consider. There are also these factors:

  • Sperm carrying “Y” chromosomes are faster
  • Age of the mother impacts the sex ratio
  • The general time of ovulation

So looking at these independently, we don’t know why Y-chromosome carrying sperm is faster. It is just something that we have all come to accept. They’re speedy, so it increases the odds that a Y-chromosome carrying swimmer will get to the egg first.
The age of the mother impacts it, and many people speculate that this ties into the birth ratio evening more towards 50/50 as a woman ages.
Finally, the general time of ovulation. No, you can’t plan out getting it on at 5:00 pm your local time in hopes for a girl. But, what day of your ovulation you conceived might impact the gender.
These factors all play a role and destroy the 50/50 concept. Predicting a baby’s gender is much more than flipping a coin.

ISo with the plain odds of 100:105 which roughly translates to 51.2% chances of having a boy. For those placing bets, the safe bet is that a younger or first-time mom will have a boy.

Many people consider these odds the best guess, but how can you tell what is going on for your baby?

One theory suggests the following formula:

49 (the chances of having a girl) – Mother’s age – Month of conception

An even number is a girl, and an odd number is a boy. So, what part does math play in this? Well, this is one of the few formulas that consider both the mother’s age and time of conception to some degree.In an example you would see:

  • 49 – 19 – 1 = 29 a boy, Or
  • 49 – 25 – 4 = 20 a girl, Or
  • 49 – 30 – 9 = 10 a girl

These of course are fun ways to predict the sex of your baby while also being able to claim that you put your hard earned math and science skills to work.

To take the most direct approach, a betting person would look at the odds, right? A boiled down version of the overly complex probability formulas.
Okay, the natural odds are that for every 100 girls born, there are 105 little boys born .   Why has nature decided this? Speculation is that it’s because men typically face many more dangers through their younger years. Historically, more men die in war than women. Historically, women live longer.
It’s likely that nature has a hand in offsetting the difference in life expectancy. From birth, without extenuating circumstances, these are the averaged life expectancies of both sexes.

The Bayesian Analysis

This probability math formula uses the following wording and might take you back to the days of long word problems.
A large container has two children, assuming that there is an equal probability that either child is a boy or girl; there are three outcomes. Both are girls, both are boys, or there is one of each. From the primary problem here there are these outcomes:

  • ¼ – chances of both being girls
  • ¼ – chances of both being boys
  • ½ – chances of one of each

But Bayes’ Theorem tries to accommodate that nature tends to favor baby boys, by using this math problem with the addition of assuming there is at least one boy.
What you end up seeing that is that the probability begins from fluctuating from the ¾ likelihood of two boys and then to 2/3 possibility of two boys when you remove the “at least” from the equation.
What does this mean for you? Let’s strip to this down to percentages as it’s easier to see a side by side comparison than fractions.
With the Bayes’ Theorem, there is somewhere between a 75% and 66% chance of a mother to have a boy assuming that nature favors boys. That’s a small window with a huge chance of having a boy! But it doesn’t accommodate for other known factors that impact the gender such as the parent’s age.

A more fun and straightforward way of looking at the odds is the Martingale Analysis. Are you a betting person? Willing to take a wager?  

Imagine that you placed a bet that someone has two little boys, with fair odds, which means that your $1 bet will pay out for $4. Now, a gambler would know that the payout of one child being a boy and both children being a boy will have different odds. It is less likely that only one child will be a boy. Remember from the first part in the Bayes’ Theorem? 

When one child is a boy, the first bet doubles, now you have $2. For a fair bet, the payout must double when the second child is also a boy, and that is how you get the $4 payout. 

The $4 payout though changes the worth of your dollar to $1.33. When you remove the betting elements and break it down into odds again, you’re looking at 1 in 3 odds of both being a boy.  

One formula with no scientific backing but purely for fun is this:

  • Numerical representation of the month of conception + 1 = 1st number
  • Age of the mother + 1 = 2nd number
  • Add the above numbers together.

For example:

8 (August) + 1=9  26 + 1=27  9 + 27= 36 – a boy!

If even, the baby is a boy. If it’s odd, the baby is a girl.
Another fun formula only uses the ages of the parents at the time of conception. This formula has some pull from the Chinese and Mayan calendar methods but has a little scientific background as well.
Because the age of the parents the probability of gender, it is not shocking to see math at work trying to support it.
This formula is really simple. Divide the number of years of each parent by 4, then whichever has the higher remainder, is the more likely gender.

  • ¼ – chances of both being girls
  • ¼ – chances of both being boys
  • ½ – chances of one of each

For example:

Mother’s age: 28/4 = 7
Father’s age: 30/4 = 7 with the remainder of 2.
From this example, the baby is likely a boy because the father had a remainder of 2.

The Actual Odds of having a Boy or Girl

ISo with the plain odds of 100:105 which roughly translates to 51.2% chances of having a boy. For those placing bets, the safe bet is that a younger or first-time mom will have a boy.

Many people consider these odds the best guess, but how can you tell what is going on for your baby?

One theory suggests the following formula:

49 (the chances of having a girl) – Mother’s age – Month of conception

An even number is a girl, and an odd number is a boy. So, what part does math play in this? Well, this is one of the few formulas that consider both the mother’s age and time of conception to some degree.In an example you would see:

  • 49 – 19 – 1 = 29 a boy, Or
  • 49 – 25 – 4 = 20 a girl, Or
  • 49 – 30 – 9 = 10 a girl

These of course are fun ways to predict the sex of your baby while also being able to claim that you put your hard earned math and science skills to work.

Why do the odds favor boys?

To take the most direct approach, a betting person would look at the odds, right? A boiled down version of the overly complex probability formulas.
Okay, the natural odds are that for every 100 girls born, there are 105 little boys born Why has nature decided this? Speculation is that it’s because men typically face many more dangers through their younger years. Historically, more men die in war than women. Historically, women live longer.
It’s likely that nature has a hand in offsetting the difference in life expectancy. From birth, without extenuating circumstances, these are the averaged life expectancies of both sexes.

The Bayesian Analysis

This probability math formula uses the following wording and might take you back to the days of long word problems.
A large container has two children, assuming that there is an equal probability that either child is a boy or girl; there are three outcomes. Both are girls, both are boys, or there is one of each. From the primary problem here there are these outcomes:

  • ¼ – chances of both being girls
  • ¼ – chances of both being boys
  • ½ – chances of one of each

The Martingale Analysis

A more fun and straightforward way of looking at the odds is the Martingale Analysis. Are you a betting person? Willing to take a wager?  

Imagine that you placed a bet that someone has two little boys, with fair odds, which means that your $1 bet will pay out for $4. Now, a gambler would know that the payout of one child being a boy and both children being a boy will have different odds. It is less likely that only one child will be a boy. Remember from the first part in the Bayes’ Theorem? 

When one child is a boy, the first bet doubles, now you have $2. For a fair bet, the payout must double when the second child is also a boy, and that is how you get the $4 payout. 

The $4 payout though changes the worth of your dollar to $1.33. When you remove the betting elements and break it down into odds again, you’re looking at 1 in 3 odds of both being a boy.

Old Wives Formulas

One formula with no scientific backing but purely for fun is this:

  • Numerical representation of the month of conception + 1 = 1st number
  • Age of the mother + 1 = 2nd number
  • Add the above numbers together.

For example:

8 (August) + 1=9 26 + 1=27 9 + 27= 36 – a boy!

If even, the baby is a boy. If it’s odd, the baby is a girl.
Another fun formula only uses the ages of the parents at the time of conception. This formula has some pull from the Chinese and Mayan calendar methods but has a little scientific background as well.
Because the age of the parents the probability of gender, it is not shocking to see math at work trying to support it.
This formula is really simple. Divide the number of years of each parent by 4, then whichever has the higher remainder, is the more likely gender.

  • ¼ – chances of both being girls
  • ¼ – chances of both being boys
  • ½ – chances of one of each

For example:

Mother’s age: 28/4 = 7
Father’s age: 30/4 = 7 with the remainder of 2.
From this example, the baby is likely a boy because the father had a remainder of 2.

Have Fun

You might enjoy math, but don’t take it too seriously! Enjoy your time spent predicting the baby’s gender and planning names without restriction. Soon enough your doctor will be able to confirm the gender of your little one, or little ones.   The surprising thing here is that there is so much science and historical backup for these formulas. When you consider static factors such as the age of the mom and dad, and the time of conception, it seems like a fool-proof formula will be here soon.

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